Remember that 100 man melee mode in smash?

That could be adapted into a neat RGM on dominion or ascension I think.

Two basic ways it could work:

5 players vs many bots. The team has to fend off waves of bots, each champion bot only appears once so you end up killing everything once. The bots could get a victory condition such as a nexus type thing in the middle, but they don't need one. The challenge could be to win as quickly as possible with as few deaths as possible. There could be achievements for this that yeild icons or key shards.

The alternative is humans on the offense and defense. Defense would need some kind of superbuff, offense would be "youre given a champ, when you die you immediately respawn on another champ, game ends when the team's died on all the champs. In this case there are two possible objectives for the offensive team. A nexus type thing to hit, or the defenders have a set number of lives and when they run out they join the offensive team. After all, it's no fun being out of the game, and losing a teammate hurts because it goes from 5v5 to 6v4, 7v3, etc, until all the defenders are out of lives.

As far as mechanics and incentives to make the mode work and feel fun, I have a lot of thoughts that I won't get into here because when making a game it's pretty easy to add rules so that it works as intended. Shopping and levelling are examples of these details.

I've been interested in the idea of a mode that cycles champs for a while (since OFA mirror mode i think, in most of those games the novelty could be accomplished in 5 mins but we were stuck in it for 25). Let me know what you think of the current iteration.