Limits of UE4 mod support

So I’ve had an idea for a game floating around for a while now, that would, in a sense, require mod support. Put simply I want to create a game, or games, that allow modders to alter just about pre-existing aspect of the game, add new mechanics, new quests, and so on.

However, I’ve been researching mods for other games made in UE4, and for the most part, there’s seldom any content that goes beyond mere model or texture replacements. So, my question is, if I give a game made in UE4 mod support, what can’t mod-creators do with a game, and are there any ways to give a game full customization-opportunities.