Well let's see... KotOr already has KotOr2 and I'm pretty sure we'll see a third.
Morrowind has Oblivion coming up.
Fable 2 had been hinted at.
Jade Empire 2: probably, but most likely developed by Obsidian
Arx Fatalis : didn't they already announce a sequel using the Source Engine?
Sudeki: I'm on the fence with this one. I picked it up used to 9 bucks, but if I had spent full price on it I would have been disappointed.
Now, which is the most deserving? Right now I'd have to say Morrowind. It had a great world with alot of openess. if the Radiant AI system they've been pimping works out as good as it's supposed to I think the series will have reached it's real potential as an immersive RPG.