This simple mod of the input.settings configuration file of The Witcher 3 rebinds the game's combat controls to a Dark Souls-like (more or less) layout.

Light attack : RB/R1
Heavy attack : RT/R2
Cast Sign : X
Item/Crossbow : Y

Everything else is left vanilla.
This is a simple modification, but it might get broken when the game updates. It is working as of 12/19/2016, game version 3.0.10.5955.

Step 1 : Download the custom input.settings file here :

https://www.dropbox.com/s/iq2507pwyin2slj/input.settings?dl=0

Step 2 : Place that file in your "Documents/The Witcher 3" folder

Step 3 : That's it, done.

Removal :Either restore a backup of the original input.settings file, or delete it altogether and launch the game, the file will be restored to default.

This works on the 3.0.10.5955 version of the Witcher3.exe, but I can't guarantee that future updates won't break it.
If you wish to customize the bindings for this game even further, you can open the file with a text editor like Notepad++, and edit it to your liking (search function will help).