Instead of us wasting a move slot for burst moves, they should act like z moves where you just need a certain moves to use them. In order to use burst moves, first you have to be in burst mode then there will be a button that you can use to use them similar to mega evolution in this game. Maybe the button can be x so it would not be the same as mega evolution.
I think limiting the number of moves that the Pokken Pokemon can hold is a way to balance them. After all, why would you ever use regular Gardevoir if there were zero drawbacks to the burst function of Red Gardevoir?
For future reference: this kind of thread should go in the Suggestion section (づ ￣ ³￣)づ