I kind of agree that Assist Trophies aren't great. For one thing, they're kind of biting the Poké-balls' shtick, and for another... yeah, they seem to be the cause of some bitterness in the fandom. I don't want them gone entirely, though, so maybe they could work a bit like the summons in Midgar? Stage-specific, able to be turned off, and without the stigma of "this is a lesser character that doesn't get to be playable." More fun stage hazard, less character that couldn't quite make the cut.