I'm not a fan of a sequel for sequel's sake, so there are plenty of games I don't need a sequel to. However, there are some games that just need to be revived (Mutant League Football), and some games that really do need another one to complete.
Metal Arms: Glitch in the System was a fantastic little title. It wasn't really innovative, but it was pure fun the whole way. My thing is, they kept introducing things that could make the story really cool, and then did nothing with them. Glitch was found with an odd marking. What does it mean? Who knows. The planet had an ancient race of aliens living underneath with some mysterious spider like robot killing everything. Any added depth? Not at all. More goes on, but there are so many plot elements they brought into the game and then did absolutely nothing with. So the game ended, and....it ended. Of course, they left it open for a sequel, too.
However, Metal Arms was the first game by developer Swingin' Ape Studios. Anyone that kept up knows that Blizzard hired these guys halfway through StarCraft: Ghost's development to complete it. Then, Blizzard simply bought Swingin' Ape and turned them into their console division. THEN, StarCraft: Ghost was canned when Blizzard decided "We're focusing on Next-Gen games only...and rather than just add some polygons to Ghost, we're going to abandon it". Thanks Blizzard.
Now, would Blizzard spend the time allowing their team to make Metal Arms 2? Somehow, I doubt it. I'm also curious if Vivendi Universal actually owns the rights to Metal Arms, as they were the publisher. If they do, then...damn.
Other than that and some already mentioned titles, I'd love to see Aliens vs. Predator 3 using the Doom 3 engine. Hell, just another Aliens vs. Predator game at all, no matter what the engine. That game kicked ass and was so much better than the movie, and from the looks of it was still better than the sequel movie coming out this December.