Well, here's a nice tip-off from BillieJoe86 (cheers): Telltale has announced, via our corporate rivals, IGN, all five games they have in development (apart from Jurassic Park and Back to the Future). You may want to sit down for this one:
Apart from Fables and The Walking Dead we first have King's Quest - yeah, that one. The classic [sic] series from 1980s and 1990s Sierra returns as a "reboot", though it's unknown if the benevolent hand of series creator Roberta Williams will be involved. Expect more details at E3.
Then we have Nelson Tethers, Puzzle Agent, returning in Puzzle Agent 2. No idea if it's a full series, but Dan Connors has mentioned, in the past, that the formula of the gameplay will change. Thanks to all you people in the comments. This will be a single episode that continues the story of the first. Here's a trailer!
Finally, Hector: Badge of Carnage - an independent episodic game with an episode (not by Telltale) already out - will see two more episodes added to its rooster.