a MMORPG for pokemon would be cool, but the problem with having it on the DS is that sometimes you are out and want to play a game, but no internet. (yes there are still places in the world without wifi)
what i think would be cool (sorry, i know this is a for XD...) would be a DS game where there was an offline section of the world where you could play, and then the online where the MMORPG part came in.
and if XD and the DS game connected so you could play your characters when on the go.