Improved Horse Controls is a stand-alone Next-Gen version of Alternate Horse Controls by KCS, ksolberg. Works on 4.04 version.
The description is a modification of the original description from the original author's (KCS) page.

-THE CONTROLS-

Reins (Steering):
Left Stick or Movement Keyboard (WASD for Qwerty) controls the reins.

Roach moves exactly to the pointing directions:

• Pull left on the reins, your horse turns left.
• Pull right on the reins, your horse turns right.
• Pull back gently on the reins and the horse slows.
• Pull back a little longer (a second or so) and you will rein your horse to a full stop. 
• Pull back when not moving and you will rein your horse to move backward. [Feature from the Next Gen update]
  

 
Spurs (Acceleration/Deceleration):
A/Cross button or Left Shift increases the spurs/speed.
(Gently) tilting down the Left Stick or Backward Keyboard (S for Qwerty) decrease the spurs/speed.

No longer need to hold a button for a full Gallop or Canter. Now the default input A/Cross button or Left Shift increases the speed and the Backward Movement decreases the speed. And Roach will maintain that speed!
The mod offers 5 speeds:

• Slow
• Walk
• Trot
• Canter
• Gallop
  

You can still long press A/Cross or Left Shift to spur directly into canter. Or double long press for full gallop (with a chance for nice animation).

Forward Spurs:
Tilting up the Left Stick or Forward Keyboard (A for Qwerty) controls the forward spurs.

While not moving, you can directly go to full Trot speed by using the moving forward command, similar to vanilla.
This will even auto-accelerate if you pull left or right on the reins.
Disables this feature will allow you to get full control of the speed.

-STEERING MODES-

Standard Steering Mode: (Default)
features options for either in "Exploration" or "Horse Racing" mode.

While riding, if you move forward and move the camera, your horse will go in the direction that the camera is facing.
If you stop holding forward, you can freely move the camera around and use the Reins to control the horse. 

Alternative Steering: (When Standard Steering Mode is OFF)

You have complete control over the camera and your horse, they are separate yet connected, they are both leaders and never follow another.
Your horse can also rotate 360° just by pressing to one side.


ADDITIONAL MOD FEATURES:

-- Controlled slow motion for high speed manoeuvring.  At full gallop, activating Witcher Sense triggers slow motion (bullet time).  This makes navigating difficult turns and timing obstacle jumps at high speed much easier.

-- Improves the timing of Geralt's voice commands to Roach.  When you say 'Whoa!', it's in the proper context.  When you yell 'Run Roach!' it's in the proper context.  Because Geralt's voice commands to Roach actually make sense, based on your current action, it no longer feels like Geralt is yelling at Roach for no reason.

EXTRA FEATURES BY THE NEXT GEN AUTHORS:

-- Add input to toggle the option "Disable Road Following". Input "9" for Keyboard & Mouse only.

-- Incorporated free-permission mods:

• Roach Never Stops, by lasyan3 (Option: Unstoppable)
• Galloping In Cities, by JupiterTheGod (Option Gallop in Cities) 
  

-- Allow to prevent Gallop speed when being encumbered, meaning the inventory is heavy (Option: No Gallop when Encumbered)
Idea by Strahlimeier.

-- Allow to instantly reach Gallop speed when long press the speed input (Option: Instant Gallop)
[Must be Enabled if you're using Ultimate Gamepad Movement]

-- Allow to disable Stamina horse (Option: Vigorous)

-- Allow to disable Fear horse (Option: Fearless)

-- Allow to disable Horse comments (Option: Silence)


Menu Options:
{
Toggle > Standard Steering Mode for Exploration on/off
Toggle > Standard Steering Mode for Horse Racing on/off
Toggle > Forward Spurs on/off

Toggle > Gallop in Cities and Settlements on/off - Takes effect upon entering a city or settlement.
Toggle > No Gallop when Encumbered on/off - Cannot gallop when the inventory is too heavy.
Toggle > Disable Road Following on/off - With an input.
Toggle > Instant Gallop when long press on/off - Must be enabled when using Ultimate Gamepad Movement
Toggle > Slow Motion when using Witcher Sense on/off - Only available in "Slow Motion" version.

Toggle > Unstoppable on/off -  A Witcher's Horse stop for nothing! Not even water, cliffs or obstacles.
Toggle > Silence on/off-  A Witcher say nothing on his horse.
Toggle > Vigorous on/off-  A Witcher's Horse never gets tired.
Toggle > Fearless on/off -  A Witcher's Horse scared of monsters, humans and beast? This can't be.
}
With three presets: Off ; Immersive ; Invisible (Enable all)

Installation:
For easy Installation, use The Witcher 3 Mod Manager. This will install everything!! (Not like Vortex).
Then use Script Merger and merge all scripts.

For Manual Installation: (If you know how)

• Extract the mod in the game's directory, where there is four folders: Bin, DLC, Content and Mods.
• Use TW3 Menu Filelist Updater.
• Use Script Merger and merge all scripts.
  

Recommended Complementary Mods:

Smooth Mount and Dismount - Fixes the stuttering bad camera transitions when mounting and dismounting Roach.
Ultimate Gamepad Movement - Improve the controller usage for walking/running.
The Stable - Horse Customisable - Add new type of shop to customise Roach.