Yeah, it's kinda overdue, but we've never been lucky with regard to getting 3rd parts from Valve, have we? HL, Portal, L4D, all arguably overdue.
Unless we allow for a "by any means necessary" provision. Given that ep1/ep2 were the only parts of HL that had proper pacing, I'll happily accept them in lieu of a HL3. Aperture Paint was an interesting break from Portals, and possibly a good alternative to "more of the same" (and then, I've still got Thinking with Time Machine
kicking about somewhere). And I do believe the team who originally did L4D is responsible for Evolve, so maybe that's where to look for a legitimate (spiritual, as game-play, if not genre, goes) successor. Outside L4D's quality of "There's nothing to know, just drop in and start shooting", I'm not sure whether I could go back from Dead Island, anyway. (The more pertinent question may be, will the upcoming Dead Island 2 seem a bit blah after Dying Light?)