This is what going through my gaming library makes me ask.
There are so many great games I've played in the past that I really wish could have gotten sequels.
The Legend of Dragoon - PS1
How the holy hell has this never gotten a sequel? Or at least a remake? It's basically Medieval Power Rangers with Dragons. It's beyond me. Everyone who had a Playstation had this game. Everyone loves it. I've played it a few dozen times at least. Although instead of a sequel, I would prefer a prequel with Zieg and his group.
Radiata Stories - PS2
I always felt that the story of Radiata Stories was never finished. We never see the end result of the war between the humans and non-humans, it just kind of falls to the side after the reveal of the big bad. Did they come to a peace agreement, did they keep fighting, what happened to Jack and Ridley in the human ending, what happened to them in the non-human ending? So many unanswered questions, and both endings to the game would have lead in perfectly to a sequel.
Psychonauts - PS2
Everything about this game is freaking GOLD. The characters, the story, the level design, the music, the powers, everything. If it was its own self-contained story, it would quite possibly one of the greatest video games ever (not that it isn't in my own top list), but it isn't. The game ends on a freaking cliffhanger. We need Psychonauts 2. We need it NOW.