The ability to fight against your own pokemon team, like Mewtwo strikes back or playing as another in game character and having to fight you
EDIT: For clarity sake, imagine you fight an impossiboss at some point and he recks you, then later in the game you play AS the imposiboss and relive that encounter but from his Point of view, where you use his team to dominate your own.
The seeds for this are present, you can clone the players party to a variable/switch, change up their party, then call upon that switch to give the player their team back, and in an old rom hack I played who knows how long ago the Mewtwo strikes back fight happens and it had MY Team. if we had a script that could change the PBS this concept is totally possible