Leech seed should kinda paralyze like it did in anime.
in a double battle, a fire attack and a water attack used in the same turn by opponent/teammate, should create a defensive heated haze, that reduces accurac and vision and evasiveness for both teammates
In the same way, fire+ice should create a water attack, like ice beam+flamethrower=a water move that is revealed in D/P that has the strength of the mentioned move, the same way that blizzard and fire blast would make a hydro pump. If used by teammates, it would combine to destroy foe! HAHAHAHAHA! If used by opponent and self, like gyrados uses ice beam and charizard uses flamethrower they would both backfire upon the foes.