I can think of a ton of games that deserve sequels. I don't know whether or not they would work out, but here's a list of my picks:
- The Clock Tower-series (4, Haunting Ground doesn't count)
- Freedom Fighters
- Chaos Legion
- Drakan: The Ancients' Gates
- Damnation (Hell yeah, I love that game.)
- Folklore
- Call of Cthulhu: Dark Corners of the Earth
- Extermination
- Eternal Ring
- Headhunter (3)
- Alone in the Dark (Hey, it has a lot of potential.)
- Legendary
- Manhunt (3)
- MDK (3)
- Metro 2033
- Project Eden
- Psychonauts
- Psi-Ops: The Mindgate Conspiracy