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One of the idea I have was that champion has category. To make it dead simple, let's say a rectangular and triangle category. You can only choose 1 from each category. 1 meant for defense and 1 meant from offense. So you don't do full offense and snowball. But, I am just trying to make it simple for now to see how people react to this new concept.

I agree definitely should be limited to one champion or maybe if two or more are used you catch a debuff penalty(s) for the battle.

Yeah, I was contemplating this, don't know why we would have 0 at bronze but put abilities at bronze, also I think limiting to one - but then moved on to why would this need a separate type just make it a unit and say "can only use 1 unit with a mission ability per battle"

You can be in gold that allow 1 champions, but for any reason you don't have enough to build that champion into gold level, so you use bronze level champion for the time being.

Reasons of why bronze allow 0 champion is for simplicity. It's very common for games to introduce complexity as the game progress, and bronze is a very early stage. Too much complexity will overwhelm players.