Hm. I wonder if having some sort of limited resource that made it so you could have only certain "amounts" of "I.O."-equivalents on a given character would help with that. Not completely solve it, but help.
Call it "awesomeness," and you have to have a certain amount of it for every I.O. you want to have equipped. You gain more "awesomeness" as you level, of course, and maybe that's something you continue to gain through "levels" even if everything else about leveling up stops being meaningful. Or maybe it's something you can get through "endgame advancement" in a non-leveling sense. But there's no "you have to be 100% I.O.s or you stink" because that's still further you have to progress beyond max. level to be ABLE to have "all" I.O.s all the time. Maybe we could even use awesomeness as our factor that determines the maximum level of "enhancement"-alike you can use; spread around your awesomeness, and you'll have a bunch of lower-tier ones for your level, but focus it all on one and you might have one bit of "gear" (enhancement-alike or whatever) that's actually beyond what a PC of your level is expected to have, but the rest of your stuff is going to suffer for it.
This might require having the ability to freely unslot your powers, which may have its own pitfalls, though.