Eternity in an hour
Personally I think something that would be pretty fair is the ability to flag a save slot as "modded". As long as you are playing on that save slot, the game behaves as though it is modified other than that you are able to save progress. You cannot collect emblems, cannot earn unlockables, etc. This allows you to play through the game as modded characters over multiple sittings in a way that is fair. You would have to reboot the game in it's vanilla state to be able to save on normal save slots again, but that's not too big of an issue.really all i want is to be able to save custom games without a patch. i understand why you can't do this currently for cheating reasons but i don't see why it can't just disable emblems and the ability to unlock content while custom content is loaded while still allowing you to save. the idea that players could use broken characters to blow through the game too easily is irrelevant since they could just do that without saving anyway.
oh and switching over to udmf maps or at least having compatibility for them. i first figured it was infeasible but apparently srb2k is going to have it in 2.0 so i can't imagine it's impossible for the main game. would really open up mapping possibilities but i understand it's a tall order.
Perhaps modded slots could even remember which character mod was saved to it so that it automatically gets loaded into the game when you select that slot if it's not loaded already?