I've always thought it would cool to get dark/steel.. and we did, yay.
I've also thought a psy/ghost would be cool too. Seems like Gardevoir or reuniclus could have done that.
Before larvesta, I thought an alternation of scyther could be bug/fire. It could be reddish colored and have glowing hot blades.
Also, I like the idea of a pumpkin style grass/fire, that could be pretty sweet.
Maybe they could do something like a geyser pokemon for a fire/water. Not sure how to make it look though.
I've always though it would be cool to have mash ups of the first gen starters as legendary grass/fire, fire/water and water/grass types.
I can't see a human-shape Fighting/Flying. What kind of creature based off a human will be able to fly?
I guess like an archangel. Something based on that would probably be a legendary, and I don't think that would fly, err... I don't think people would like that.
fighting/flying could be a bird with a temper problem. it could even be modelled after one of the angry birds and explode lol. Psychic/Dark doesnt seam like it would necessarily be a light/evil kind of pokemon because dark is more represented as dirty fighting in the translation.
I've always that farfetch'd could easily have been fighting/flying and girafarig could have been psy/dark. Also, although it's not real I've always imagined mewthree as a psy/dark type.