I've really never done a lot of online multiplayer, except for losing a few hundred hours to quake live (had to delete it off my computer finally...really fun, but too distracting and addictive).
That being said, racking my brains, the two games I'd love to see would be some kind of turn-based strategy like advance wars/fire emblem, and some kind of multiplayer bullet hell game. Now either of those would be awesome. Also, I've always thought something like a MMO 2d GTA would be really cool, but being able to really wreak havoc like destroying buildings, massive car pileups...just total chaos.