I haven't really played a Pokemon game in a while and maybe there is a move like this already in one of the generations or as one of the special trait abilities or as a held item, but I would make a move where you "Sacrifice" all of your Pokemon's HP and it goes to 0, but that Pokemon becomes invincible to all damage for a few turns (like 1-3 or the count is random like Magnitude). After that random set of time the Pokemon dies instantly. Also when its at 0 no HP items/abilities can be used and you cant swap the Pokemon out.
Probably overpowered/underpowered depending on the circumstances and would just be some kind of last ditch effort move or a tactic if used with reserve characters and like I said maybe there's something like this already in the games that I don't remember, I just think it would be a pretty cool idea.
Also maybe some ability where you can transfer HP from one of your reserve Pokemon to the one currently out in combat(Blood Transfusion/Transfusion), or a special ability trait that has them constantly leeching a small amount of HP from all your reserve Pokemon every turn(Parasitic).
Likewise maybe a mon-trait that has them constantly dealing a small amount of damage every turn passively to them self and the opponents Pokemon (Volatile/Unstable).
Also even though the three Porygon's have Conversion I think a cool unique ability to them (and in the .1% chance they make MissingNo an official mon for shits and giggles) would be "Dimension Shift" -The Pokemon reverts to a 2-dimensional form and severely lowers its normal damage dealt/received, but also drastically increases the special attack damage dealt/received. Using Dimension Shift again returns it to a 3D form and reverses the effects and maybe can only be used every few turns.
Also for shits and giggles since it references this in I think Ruby/Sapphires Pokedex entry for Porygon: "Copy-Protection" -This Pokemon is copy-protected and an opponent can not use transform or the move Copycat and Mimic on it.