Grim Fandango - either a prequel using Manny in some other storyline with a mostly new cast (except his coworkers) or a sequel using an all new cast or some clever reuse of the old ones. Hell, I'd settle for a remake with more modern graphics and much better controls.

Beyond Good and Evil - one of the great games. Actually this is getting a sequel finally, but I thought I'd list it anyway.

Bone - Telltale gave us the first two parts and then just stopped and switched to Sam and Max and Strong Bad.