Some other type combos I would like to see:
Ice/Poison - Often considered the 2 worst single types in the game, I would like to see what putting them together would do. Offensively they cover too much of the same ground, and defensively they don't provide anything extra. I just want to see if this type combo could possibly work at all.
Rock/Dragon - I think it would be cool if a pseudo-legendary was a fossil Poke that you find late, or end game. Another option is to have it based off a Gargoyle.
Psychic/Ghost - A pokemon with absolutely no physical presence. Super high special stats. Okay HP, and speed. Piss poor Attack, and Defense. Give him Calm Mind, and Nasty Plot and just let him go wild.
Psychic/Dark - My idea for this is to make him the Pixie for this gen. Make him somewhat similar to Mew (though without the whole learn every TM thing) and give him Prankster.
Bug/Dark - Based off a cockroach, and has Speed Boost as his ability. Make him super bulky and give him decent Speed (I'm thinking somewhere between 76 and 84).
Grass/Fire - This is actually a Pokemon idea that I've been working on for a while. It's a Jack-o'-lantern with high special attack. It's 2 abilities are Solar Power and Chlorophyll. Basically, make the ultimate sun abuser.
Water/Fire - Preferably based off the Pistol Shrimp. I don't even know how well this would work, but I want to see it happen.