Just a throwing this out there, as if we were chatting over beer / coffee / generic beverage …
How about introducing a “cooldown” on characters, inversely proportional to the damage taken in a match? So if a character came through unscathed, they could get back in the fight right away. Otherwise, the character would have a “cooldown”, say 2 seconds per (% of max hit points) damage taken. So if buddy comes out of match at 50% health, he/she/it would have to wait 100 seconds before the could start the next match.
The cooldown would start as soon as you return to the node opponent/selection screen. A healing pack will take one back to 100% health, and immediately negate remaining cooldown.
The idea behind this, is that in a game where speed wins, a player would need at least two competitive rosters to play continuously, or wait a minute or three before they roll out the A team again. This would apply to all renditions of a given character (ie so if you have 2 X-force (!), neither of them could play during cooldown.