Most of stuff you've listed is impossible either because of lack of tools or engine limits. Perhaps we'll get rid of those engine limits once the final Volition's modding patch is out but I don't think there will be a way to create new missions, at least not in this decade. Bringing back fighting styles is something we only can dream of. Putting NPC outfits on the player is possible but it's extremely buggy (especially with female characters) + you'd have to make DLC homies outfits unlock only for people who own it. We can't create new cars nor NPC models. And there's plenty of mods which make gangs spawn on the street already.
EDIT: I'm also wondering what kind of bonuses you're going to add to achieve 555+ amount, there's very few bonus types in the game.