I think I'm going to name a few common ones or ones already named, but here goes...
No One Lives Forever 3:
Hell, like many of us here, I would even be happy with just a re-release or updated version of the original.
Anachronox 2:
There was a lot of cut content from the original that would have made its way into a sequel (which due to dismal sales never materialized)
SiN 2:
Nothing episodic but an actual full-length game.
Freedom Force 3:
I heard about how the series would have followed the tonal and stylistic trends of the various ages of the comic book medium. It would have been neat to see a gritty age of the Freedom Force world. And that damn cliffhanger.
Daikatana 2:
I'm going to be in a minority, but aside from the "adventures in babysitting" that was managing your AI partners, I would like to see what other time periods Romero would have had us visiting. Like those guys in Pulp Fiction, I think the makers of Daikatana acted with the best of intentions.
LA Noire 2:
I'm a sap for games set in the first half of the 20th century, especially with a noir ambience. LA Noire was a bit clunky, but maybe it could be better the second time around.
Sherlock Holmes: Crimes and Punishments 2:
I think they finally got the formula right this time.
The Wolf Among Us:
I wasn't really sold on Sam and Max, but The Wolf Among Us was my first bite of Telltale goodness.
Ultima IX:
Or better yet, the original version we were promised.