I've been having that idea in mind as well. Not necessarily as something I really would want to push for, but from the point of view of game design. Because getting a "bonus" for Backstab does not exactly feel rewarding.
Compare with Barrelmancy. Ok, it is aesthetically weird in medieval-renaissance fantastic Faerun, and it isn't super immersion-enhancing that nobody cares when you fill the room with barrels. But in order to Barrel-kill Dror Gagzlin with Roah's barrels, you need to have found the stock of barrels, think about the idea (imagine a first-time player who hasn't been on the forums), and then carry the barrels. You have to work for your firework.
Backstab ? You walk around the enemy, you Backstab. Every turn. Zero effort. It's basically default mode. Calling this a bonus at the moment feels meaningless. It's nearly a reward for having launched the game.
So, yeah, from this perspective ... why not just make the "bonus" part of the core rules and be done with it ?
I mean, if Larian is so concerned about the miss rate and they started "tweaking" the 5E rules to make a homebrew ... why not just running with it ? In for a penny, in for a pound as they say.
