Just here me out.
16 players = better
Call it Flood King or something cool.
Its Like Regicide, but only whoever is the king is the flood. When the king kills a player, that person becomes the flood but when it kills, it just gets points. Same with non-flood Spartans.
When the flood that was converted by a king earns enough points, it becomes the king and stays the flood. But it then in turn converts non-flood players to flood and then if it kills another flood, that flood becomes a spartan.
Winner is whoever has the most points/king by the time all players are converted to flood.
So, if any questions, feel free to ask, but please this or add custom game-type creator in Halo 4, thank you…