I know about Rotom's formes, but they only add an specific move, so I'd rather see one Electric/Fire that at least allows me to get STAB Thunderbolt and Flamethrower. And the same for Electric/Ice (STAB Boltbeam) and Electric/Grass (at least so it can use Energy Ball or Leaf Blade or whatever).
Other than that, I'd like:
Fire/Water Fire/Ice (screw the metagame; yeah, you read that right) Fire/Grass Ice/Fighting Electric/Fighting Electric/Psychic Electric/Dark Grass/Ghost Grass/Dragon Psychic/Dark Ground/Fighting Rock/Dragon
But, any other type combo we haven't seen yet is fine with me.