I've toyed with this idea for a while now. Hypothetically, I could see it being essentially a last-man-standing situation. Blowing up one nexus doesn't end the game, it's the last nexus remaining that's the winner. So that even if Team A loses their inhibitor, they still get to fight against Team B and Team C while Team B & C are duking it out for the win.
It would HAVE to be a non-ranked game mode, of course. And there needs to be some kind of incentive for teams B & C to keep pushing Team A, or there won't be any point to them making a come-back...
I dunno. It's fun to think about, but I can't really see it being a serious mode at all, if it could even really be implemented.