Also by me:
Free Combat
New Game Plus Plus

What does it do?
On one hand makes it possible to start a New Game + using a New Game + save.
It is also possible to start a New Game + using a New Game ++ save, i.e. stacking New Game +. Thanks to Enpror for the confirmation :)
(The console command NGPCStandardInit might not work for this purpose but NGPCCustomInit and NGPCCustomInitResetDiagrams should.)

Additionally it can:
- raise the player level cap
- scale enemies/crafting schematics/quests (suggested levels)/new equipment
- disable the equipment level requirement
- level equipment with the player
- add a damage taken limiter
- add a damage dealt limiter
- increase merchant funds.

On the other hand it improves combat:
- The vanilla game's combat stance doesn't allow running, only sprinting, which costs stamina. The mod adds combat stance with running enabled and normal stance (all of them selectable mid-combat). The setting is called Enemy strafing and its options are:
  1. On, running disabled - vanilla game behavior, Geralt fights in combat stance and can't run;
  2. On, running enabled - Geralt fights in combat stance and can run;
  3. Off - Geralt fights in normal stance (like outside of combat) and can run.
- Supports jumping (toggleable mid-combat).
- Enables looting (toggleable mid-combat, though delayed).
- Allows setting enemy level below/at/above player level by a desired amount (can also be randomized).
- Adds a combat animation speed multiplier (range: from normal to twice as fast).
- Adds a chance governed damage dealt multiplier for too weak/strong enemies, independent of gameplay difficulty (adjustable mid-combat, range: from half to double damage).
- Adds a chance governed damage taken multiplier for too weak/strong enemies, independent of gameplay difficulty (adjustable mid-combat, range: from half to double damage).
- Allows setting the chance of weapon degradation (100% without the mod, at 0% weapons don't degrade).
- Allows setting the chance of armor degradation (100% without the mod, at 0% armors don't degrade).
- Can speed up, slow down or pause the game.
- Just for fun: makes it possible to taunt the enemy at will.

How to install?
1. Unpack the downloaded .zip file to the [Witcher 3 main folder].
2. Add the following key bindings to the [Combat] section of C:\Users\[User]\Documents\The Witcher 3\input.settings:

    IK_8=(Action=ToggleEnemyStrafing) - cycles through all three Enemy strafing options
    IK_D=(Action=ToggleEnemyStrafingCombat) - only toggles between Enemy strafing with and without running (the combat stances)
    IK_Q=(Action=ToggleJumping)
    IK_W=(Action=ToggleLooting)
    IK_Space=(Action=CbtJump)
    IK_A=(Action=CbtTaunt)
    IK_9=(Action=Speedup)
    IK_0=(Action=Slowdown)
    IK_P=(Action=Pause)
    IK_NumPad4=(Action=DecDamageTakenMultiplier)
    IK_NumPad5=(Action=IncDamageTakenMultiplier)
    IK_NumPad7=(Action=DecDamageDealtMultiplier)
    IK_NumPad8=(Action=IncDamageDealtMultiplier)

    and change them to other keys if you wish. You don't have to add all of them, only those you intend to use.
3. Only for Next Gen (NGPC 2.60+): for the mod menu to appear insert
    modnewgamepluscombat.xml;
    at the end of
    [Witcher 3 main folder]\bin\config\r4game\user_config_matrix\pc\dx11filelist.txt (or dx12filelist.txt,
    depending on which game version you use).

If using multiple mods, you will likely need to merge them with Script Merger.

How to uninstall?
1. Delete modnewgamepluscombat.xml from [Witcher 3 main folder]\bin\config\r4game\user_config_matrix\pc.
2. Delete modNewGamePlusCombat from [Witcher 3 main folder]\mods.
3. Remove the key bindings from C:\Users\[User]\Documents\The Witcher 3\input.settings.

How to use?
IMPORTANT: If you want to level above 100, you must also install Fixed Level 100 Plus (Next Gen) or Level 100 plus - No quest item and treasure fix (pre Next Gen, thanks to Alexthegr81 for the heads-up) for looting to work properly. Some quests - including main quests - will break without it.
Alternatively, you can just leave the level cap unchanged in the menu, at 100.

Step by step guide:
1. Load an existing NG+ save.
2. Open the console by pressing "~" and type or copy-paste one of the commands NGPCCustomInit or NGPCCustomInitResetDiagrams or NGPCStandardInit. The game will appear to freeze, this is normal.
3. Save in an empty slot - don't overwrite existing saves!
4. Quit to the main menu.
5. Start a New Game + and select the new save.

If you don't have a console mod installed, add DBGConsoleOn=true to the [General] section of general.ini in [Witcher 3 main folder]\bin\config\base.

Console commands:
NGPCCustomInit
Runs custom initialization that only removes inventory items that would automatically start quests if kept.
Use NGPCCustomInitResetDiagrams instead if you want to complete the treasure/diagram hunt quests.

NGPCCustomInitResetDiagrams
Same as NGPCCustomInit except it resets the crafting schematics, enabling you to complete the treasure/diagram hunt quests.

NGPCStandardInit
Runs the standard New Game + initialization which removes certain inventory items, resets crafting schematics, etc.
It may downgrade your equipment. Use a custom init command instead if it does.
Workaround for downgrade by 9SMTM6: place your armors/swords on mannequins/weapon stands in your Corvo Bianco vineyard home before executing the command and equip them afterwards. Simply dropping them, executing the command, then picking them up might also work (not tested).

NGPCLockVanillaEquipmentLeveling
For Equipment leveling: Vanilla only.
Locks inventory items. If using Vanilla Equipment leveling a piece of equipment becomes (very) high level when placed in your inventory, try these steps: 
1. Set Equipment leveling to Off in the menu
2. Place the item in your inventory
3. Execute NGPCLockVanillaEquipmentLeveling in the console
4. Reset Equipment leveling to Vanilla.
Simply executing it will lock all inventory items at their current levels while newly found items will be scaled based on the New game plus combat level.

NGPCUnlockVanillaEquipmentLeveling
For Equipment leveling: Vanilla only.
Unlocks inventory items. The opposite of NGPCLockVanillaEquipmentLeveling.
Equipment can easily be overleveled with it. Use with caution.

NGPCClearOriginalEquipmentLevel
For Equipment leveling: Match player level and Equipment leveling: Restore original level only.
Setting Equipment leveling to Match player level saves the current level of an equipment item before scaling it. The saved level is then used when switching to Restore original level.
This command will clear the saved level so it can be set again. Use with caution.

In Options > Mods > New Game Plus Combat you can use:
- Maximum player level to raise the player level cap (up to 999).
- New game plus combat level to scale enemies (if Enemy upscaling is disabled), crafting schematics, quests (suggested levels) and new equipment. Adjusting it will have an effect on enemies at spawn time, so if you're in the middle of a fight, you need to move away far enough so that they are respawned; or simply adjust the slider, save, and load. It is in fact the normal New Game + starting level but can be customized at will, at any point during the playthrough (unlike in normal New Game +). Cap: 999.
- No equipment level requirement to enable equipping high level gear.
- Equipment leveling to control equipment level as follows:
  1. Off - no mod or vanilla leveling
  2. Restore original level - mod leveling: immediately restores original equipment level for equipment leveled up with Match player level
  3. Match player level - mod leveling: immediately levels up equipment with a level below that of the player, leaving higher level equipment as is
  4. Vanilla - vanilla leveling: scales new equipment when added to the inventory, based on the New game plus combat level. If set to a high value, the equipment will also have a high Required level.
- Match player level for all equipment, including high level to have Equipment leveling: Match player level scale all equipment to player level, not just items that are below it (meaning high level equipment will be scaled down).
- Prefer damage limiters & Maximum damage taken (% of full player health) to set the maximum amount of damage taken as a percentage of full player health.
- Prefer damage limiters & Maximum damage dealt (% of full enemy health) to set the maximum amount of damage dealt as a percentage of full enemy health.
- Rich merchants to have merchant funds proportional to the player level. Merchants will receive funds proportional to your level when they are initially spawned. If the option is enabled after that, they will obtain the scaled funds once a certain number of in-game days have passed (when their funds are replenished).

Options related to Free Combat:
- Configure Enemy strafing, Jumping and Looting, then load a game or start a new one. You can also press (mid-combat) the hotkeys for Enemy strafing repeatedly to select a stance, or press those for Jumping and Looting to toggle them (toggling Looting is delayed, unlike setting it via the menu and loading a game).
- Enable Set enemy level relative to player level and adjust Enemy level relative to player level to control enemy level: below, at or above player level (negative value, 0 or positive value). Enabling Randomize enemy level relative to player level will randomize it. Enemy upscaling should be disabled.
- Enable Prefer damage multipliers, then:
Raise Chance of dealing less/more damage (%) above 0 and adjust Damage dealt multiplier (x) to decrease or increase damage dealt.
Raise Chance of taking less/more damage (%) above 0 and adjust Damage taken multiplier (x) to decrease or increase damage taken.
Use the bound hotkeys to adjust both damage multipliers on the fly.
- Set Chance of weapon degradation (%) to control weapon degradation.
- Set Chance of armor degradation (%) to control armor degradation.
- Set Combat animation speed multiplier (x) to adjust combat animation speed.
- Press the bound hotkeys to jump or taunt the enemy.
- Press and hold the bound hotkeys to speed up, slow down or pause combat.

Some options are only available in-game (not in the main menu).

Tip: if you have the Ekhidna decoction, each jump will restore some vitality.

Version history:
3.80: added default (English) texts for all languages
3.70: updated to patch 4.04
3.60: minor improvement to Match player level and Combat animation speed multiplier
3.50: removed a script (easier merging)
3.40: fixed slowdown when using Blizzard
3.30: improved Match player level/Restore original level
3.20: improved Set enemy level relative to player level
3.10: updated to patch 4.03
3.00: removed a script (easier merging)
2.90: updated to patch 4.02
2.80: updated to patch 4.01, menu options set to vanilla defaults, fixed required level not hidden for armors while No equipment level requirement enabled
2.70: damage processing adapted to Next Gen
2.60: updated for Next Gen (4.00), shrank/reorganized code, might make merging easier (New Game Plus Plus 4.90)
2.50: synchronized required level shown in inventory and crafting for witcher and relic gear (New Game Plus Plus 4.80), streamlined equipment leveling (New Game Plus Plus 4.70)
2.40: fixed more cases of lasting freezing caused by Match player level (New Game Plus Plus 4.60)
2.30: improved damage processing, removed a script (Free Combat 4.00)
2.20: consolidated variants to Normal and GOG GOTY, removed a script, Equipment leveling is set to Vanilla when starting a new game (no more low-level new equipment at the start of the game) (New Game Plus Plus 4.50), New game plus plus level is set to player level when starting a new game (New Game Plus Plus 4.40), New game plus plus level should not reset to player level on load (New Game Plus Plus 4.35), New game plus plus level is set automatically (no more low-level enemies at the start of the game) (New Game Plus Plus 4.30), added option Match player level for all equipment, including high level (New Game Plus Plus 4.20)
2.10: Maximum player level is set to player level if dragged below it (prevents confusing and freezing the game) (New Game Plus Plus 4.10)
2.05: more changes to playerWitcher.ws to make it easier to merge (New Game Plus Plus 4.05)
2.00: fixed very elusive freezing triggered by Equipment leveling option Match player level (New Game Plus Plus 4.00)
1.90: init commands now clear buff immunities, preventing a bug during the Missing Son contract (and maybe other quests as well), as long as Geralt doesn't become immune to hypnosis again, beforehand (New Game Plus Plus 3.90)
1.80: added command NGPCClearOriginalEquipmentLevel (New Game Plus Plus 3.80)
1.70: Restore original level greatly improved, further improvement to Match player level (New Game Plus Plus 3.70)
1.60: Equipment leveling option Match player level more accurate (New Game Plus Plus 3.60)
1.50: enemies no longer capped at Maximum player level + 5 while Set enemy level relative to player level enabled (Free Combat 3.90), added commands NGPCLockVanillaEquipmentLeveling and NGPCUnlockVanillaEquipmentLeveling (New Game Plus Plus 3.40), made playerWitcher.ws easier to merge (New Game Plus Plus 3.45), improved Equipment leveling option Match player level for relics at high player level (New Game Plus Plus 3.50)
1.40: added Equipment leveling option Vanilla (New Game Plus Plus 3.20)
1.30: added option Randomize enemy level relative to player level (Free Combat 3.80)
1.20: added option Rich merchants (New Game Plus Plus 3.30)
1.10: upgraded New Game Plus Plus to 3.10 (lower minimums for Maximum damage taken and Maximum damage dealt (10%), a bit faster equipment leveling code); may fix freezing
1.00: initial version based on Free Combat 3.60 and New Game Plus Plus 3.00

Tested with:
Next Gen.

Credits:
Andrzej Sapkowski for creating the Witcher universe and CD Projekt Red for the games.