Mawile. Finally, something to set it apart from its fellow Steel-types (other than Intimidate). It would be the only Steel-type (besides Dialga) that could have a STAB that is super-effective against Dragons.
I kind of want Mew to be a Psychic/Fairy and the new Mewtwo form being also Psychic/Fairy or even pure Fairy. Wouldn't that be interesting? Mewtwo "awakening" a completly new side of him. He may be a "superclone" yet he's still "incomplete", so while Mewtwo would have two forms with different powers/typings, Mew the original simply has both.
Would just be interesting and would propably make it easier for some people to like the Fairy type more.
Alteria: dragon/fairy - the only dragon that wont break the game with this OP typing
Reuniclus: Psychic/Fairy or just pure Fairy
Absol: dark/fairy (I'd cried rivers with tears of bliss)
Clefairy line: pure fairy
Why would Reuniclus be Fairy? It's just based around dividing cells, and a split personality that coordinates Psychic attacks.
As for the others, I kind of see Dragon/Fairy as the new Dark/Ghost, it may still have a weakness, but it also is a sort of "ultimate" type combination. Whatever pokemon gains that type would probably need to be on par with Sableye or Spiritomb for that reason.
Absol, while I could somewhat see it, it doesn't seem quite fitting enough. To be quite honest, while I know it would never happen, Absol strikes me as a Dark/Normal. The combination of it's dex entries, which suggest that it is misunderstood, and not much of a trickster or doombringer itself, and the fact that it relies on very heavy hitting physical attacks makes me think more of Normal than Dark. If that did ever happen, it would get absolutely wrecked by Fighting though.
I could sort of see Sawsbuck, due to it being so nature based, but it does already have a unique type combination. We don't have enough Normal/x type combinations still, almost every normal type is pure normal or normal/flying.