In Battle Royale you can use Parappa and stand in a corner pulling out his boom box to gain AP(which he gains very quickly)and use his level3 which immediately kills everyone on the screen. This ability might be only limited to parappa but who knows at this point. Sly cooper can turn invisible the whole match, hide and steal AP,which he is somewhat intended to do, the whole match. My point is not every character has to physically be fighting in order to gain AP.
I'm also confused as to what you think my idea will mess up.
1) Have you seen videos of Parappa's boom box? It builds up AP at a very low pace, and on top of that it's an offensive weapon too. It lures other players to get the AP balls, but it actually stuns enemies when they approach. And Sly cooper's invisibility still requires him to be fighting and interacting with characters on the map. So yeah, characters do need to interact with each other to gain AP.
2) I already explained it. Your system would punish players who sustain a high amount of damage. There will be 2 types of people who will have taken lots of damage: weak players who are taking lots of damage and building up no super, or people who are just naturally accumulating more damage as they fight more. The former group of people are already at a disadvantage; they're not building up super. Makes no sense to hurt them even more. For the latter group of people, under your system, they would be punished for doing exactly what this game's combat system encourages players to do: build up a high super. This game's combat system is supposed to reward good fighters who build up their super with devastating super attacks; your system punishes them. That's the problem.