For a 2D beat-em-up, the issue really is the amount of animations you'd need. OpenBOR isn't that hard to learn compared to other game making software (I used to know it myself, but that was a long time ago and I assume a lot has changed). It's theoretically possible to edit Streets of Rage 4 characters, but again, that's all about the number of sprites that would need to be created. As far as editing 3D games, I'd probably look into whether the models in the PC version of Yakuza 0 can be edited. Whatshisname (sorry) already has replaced generic enemies with women and they already kick. Question would be if their outfits can be modded in Blender or elsewhere and then be replaced in-game.