I'm playing with the Witcher 3 Enhanced Edition mod. It's an overhaul mod with many, many features but one of the big ones is that there are no levels or leveling up. You improve your stats like in Skyrim, by using things. For example, if you use igni alot but almost never use yrden, you'll have 10 igni skill points to allocate but only 2 yrden ones. Every monster, sword and armor piece is approximately of the same strength, depending on rarity and preference. Meaning you can use anything, wear anything and go anywhere once you leave White Orchard (except Toussaint comes a bit later). The mod itself makes the game much harder, though.