NEXT GEN VERSION HERE: https://www.nexusmods.com/witcher3/mods/7463

XP rewarded in quests wasn’t changed. List of quests that were fixed:

• Now or Never (first half of the quest in vanilla is set to level 11)
• King's Gambit (the whole quest was set to level 16 in vanilla)
• Pyres of Novigrad (some parts in vanilla were set to levels 7 and 9)
• Echoes of the Past (the whole quest in vanilla is set to level 13)
• Novigrad Dreaming (this quest was originally set to level 10, so I changed only the suggested level that you see in the journal from level 7 to level 10 to be in line with Pyres of Novigrad which is the previous quest)
  

BONUS FILE:
I also included an optional file with suggested level changes for A LOT of other quests. I did increase the levels in reward.xml, so you should earn full XP if you're not 5+ levels above the newly changed suggested levels. I made it seperate, because I believe that everyone might not like this change, so feel free to try it if you'd like. Changed only for quests where there is a bigger level difference from a chronologically earlier quest and where it made sense to me. In the case of Skellige quests, quest levels were increased if they were lower than level 16, because suggested level for Destination: Skellige is 16. Fistfights weren’t changed because they aren’t affected by ±5 level difference.
Note: With the exception of Berengar’s Blade and Rough Neighborhood, I didn’t change levels for enemies to accommodate for the increased levels of the quests that I changed. You don’t get that much XP from killing enemies, but it takes a lot of time to find and change all the parameters in all of these quests for enemies and IMHO it’s not really worth the time. You can always turn on Enemy Scaling for a challenge, so I hope you won’t mind that.

IMPORTANT: If you plan to use the mod for changing the suggested levels, then DON'T install the main file, but only the optional file. The fix for quests mentioned on top of the page is included in the optional file as well.

List of changed quests (number in brackets is the original level and the number behind the new level):

• Haunted House (7): 10
• A Favor for Radovid (9): 12 (actually set to lvl 12, but not in the journal)
• Missing Persons (15): 18
• Nameless (14): 18
• Blood on the Battlefield (20): 24 (actually set to lvl 24, but not in the journal)
• Through Time and Space (26): 28
• Skjall's Grave (28): 30
• Honor Among Thieves (9): 12)
• Rough Neighborhood (10): 26
• The Gangs of Novigrad (9): 12
• Berengar’s Blade (27): 19 (The Isle of Mists is the cut-off point, now in line with Ugly Baby)
• Stranger in a Strange Land (14): 17
• An Unpaid Debt (15): 18
• A Hallowed Horn (12): 17
• Flesh for Sale (12): 17
• Following the Thread (11): 17 (you get to Skellige after level 16)
• Free Spirit (13): 16
• From a Land Far, Far Away (13): 16
• Master of the Arena (14): 17
• The Cave of Dreams (14): 17
• The Nithing (14): 17
• The Price of Honor (14): 17
• The Family Blade (15): 18
• In Wolf’s Clothing (15): 18
• A Bard's Beloved (15): 18
• Armed Assault (15): 18
• For Fame and Glory (15): 18
• The Last Wish (15): 18
• *NEW* Black Pearl (13): 16 (Please note: check the section Conflicts for new info)
  

*IMPORTANT UPDATE*
Because of my stupidity, I've introduced a bug with the optional file. It prevented you from obtaining A Miraculous Guide to Gwent and Zoltan gwent card from the scholar in the inn in White Orchard in the beginning of the game. This should be now fixed, so please download the new file to fix this issue. If you are not using the optional file, then you don't need to download anything again. Sorry for the inconvenience. Special thanks to Arkwulf for identifying this bug. Thanks!

*IMPORTANT UPDATE 2*
I've only recently realised that the bug with the missing Zoltan gwent card and Miraculous Guide to Gwent book (that I've introduced with this mod and was present in the optional file as well) is also present in the main file modFixForLowXP. That's why I'm rolling a new update and now it should be fixed for the main file as well. Sorry for the trouble.

Cause of the Bug:
These quests had different levels set for XP rewards from their suggested levels. So I only changed the levels in rewards.xml to match with their suggested levels and that was it.

Installation:
Simply put the mod folder in your Witcher 3 directory/mods. If this is your first mod, then just create a new folder called "mods" and put it in there. Mod Manager will probably work too. To uninstall, simply delete the mod.

Conflicts:
If you have the Witcher Book Collection mod installed, you’ll have a conflict, which cannot be resolved automatically with Script Merger. You have to use the line from the Witcher Book Collection mod AND you’ll have to change the parameter “level=11” to “level=14” otherwise this fix won’t work when you kill the katakan in the sewers during Now or Never. If you install the optional file, you'll have 3 more additional conflicts if you have the Witcher Book Collection mod installed that must be fixed manually. Again, you have to change the parameter “level=12” to “level=17” for mq2001_holmstein_completed, “level=15” to “level=18” for mq2001_kaer_trolde_completed and "level=13" to "level=16" for mq3031_2_finished_quest and then save.

Special Thanks:
A big thank you to paulr0013 who figured out where to change the suggested level in the journal.

If you think that you know about more quests that suffer from this issue, let me know and I'll look into it. Hope you enjoy!