Seconding a lot of choices on here.
Saboteur 2: The first was an overlooked gem. Too bad EA has abandoned it like a baby on prom night.
Actraiser 3: I hated the second for taking out the world building and bringing in the dumb flying/sword/shield fighting. Make it more like the first and you've got a deal.
Bully 2: Such a fun game. I'd love to see what happened after Gary got his ass kicked and expelled.
Brutal Legend II: Take out the shoehorned in tower defense stuff, and I'm all in.
The Darkness 3: Didn't play the first, but I rented the second on a whim and loved it. So i'd be all for another game with Jackie and his shadow imps.
As for my own nominations, I'd blow up a bus full of nuns and orphans to get sequels to Alpha Protocol, SoulBlazer, Shadowrun, LA Noire, Shatterhand, Kabuki Quantum Fighter, Shadow of the Ninja, Cybernator, Illusion of Gaia, Gargoyle's Quest/Demon's Crest, Dante's Inferno, and an actual Phantasy Star V, not more of that PS Online garbage.