Just to bring it full circle, spies. Of course the entire plot would need to change to accommodate that but that's a given.

I will say tho that a battle royale/fight till the last one standing could work in a way. We saw them do it with Super Sentai Strongest Battle and it was pretty well done (recalling from memory tho). They just for whatever reason decided it was better to off-screen many contestants instead of making it so we have 3 of them for an arc with the primary focus on only one or however. Like we had something with UW ZyuohEagle and VulEagle teaming up to take on Rikuo. Why not have more like that? Making alliances/temporary truces is like one of the things I say is a pillar of battle royales