I think that co-op games in general have been pretty scarce lately. Other than Hunter: The Reckoning, Halo, and Baldur's Gate: DA, there haven't been many co-op games.
Remember Guardian Heroes? That was a pretty cool co-op game. I'd like to see more stuff like that, but with maybe a little more RPG in the mix and more persistence.
One thing I'm sick of - elemental powerups. Every freaking RPG, platform, and adventure game is based around the same tired old - earth, fire, wind, cold, blah, blah, blah concept. That's one of the cool things about Hunter: The Reckoning, as it doesn't conform to this tired standard.
Animal spirits could be a cool paradigm and I'm sure there are plenty of others. Maybe signs of the Zodiac or real chemical elements of the periodic table - some sort of molecular construction powerups or something where you can manipulate more complex atoms as you go. Just a few thoughts.
I think an 8-player online super-hero game could be pretty cool. Everyone creates their own superhero to form a supergroup and then the game could generate Diablo-like quests and scenarios to be defeated. New quests could be introduced sort of like issues a comic book and downloadable for a couple of bucks or something.
I think online should go more in the direction of groups of 8-16 players, rather than the massively-multiplayer route. I think this would provide for a tighter experience.