However if there were higher lvl supers I think that no one would survive it lol
Its not a bad idea for characters like Parappa and Spike who can only get 3 kills with their level 3. Ok its guarenteed, but with some skill I think every level 3 can guarentee 3 kills.
Tag supers sounds like an interesting idea.
If there was a level 4 super, there would need to be heavy penalties and restrictions. For instance, you'd have to use it with at least a minute left in the match, and and you'd be unable to perform any attacks for several seconds afterwards.
This is of course assuming a level 4 would virtually guarantee 5 or more kills.