The Crusader series (No Remorse, No Regret) could have been built upon, with its famously cheesy live-action cutscenes.
The Quarantine series disappeared after its second installment, Road Warrior. Come to think of it, most mainstream vehicular combat games went away, like the Interstate series.
The Discworld adventure games (1, 2, Noir) were famously convoluted, but they did a pretty good job of adapting Pratchett’s world into video games.
Finally I would have liked to play the initially planned sequels to Advent Rising. I have (probably rose-tinted) fond memories of that game, but hey, you asked.