I have been doing a few, Like a Dragon series, Witcher 3, Batman Arkham Origins, FIGHTS IN TIGHT SPACES, in arkham knight I've swapped just enemies from a challenge stage, in origins even the voice of the characters were swapped to that of females.I have been having a lot of fun with Witcher 3 by making custom animated counters and finishers.
I did assassin's creed and watch dogs series long ago before losing a hard drive, but I don't recall how I did as it required messing with hex values.
Making model swap ain't really that hard, swapping logic is a bit complicated tho, still, I don't think there are many mods for everything because of the lack of interesting people have in zako in games in general tbh, or at least not when it's modded.
Modding unreal 3 (Bulletstorm, Deadpool, Lollipop, Arkham Series, etc) games is rather easy.
I'll probably be releasing the stuff soon like always as soon as they are bug free.