You had me at 'Thief-inspired', isometric RPG Seven, but 'ex-Witcher 3 devs' pretty much seals the deal. As Phil explained last year, Seven is a game set in a "non-linear sandbox world", where you'll run and jump and climb and sneak and all the other things mysterious travellers sometimes get up to in post-apocalyptic worlds. Sorry, in "beyond post-apocalyptic" worlds, which I guess is shorthand for 'has fantasy elements too'.
The first trailer for Seven has emerged, via IGN, and it's a doozy, presenting some lovely isometric environments seemingly full of detail and life, interrupted occasionally by flying robo-tentacle monsters. How like life. I particularly like the parkour element—you'll bound across rooftops and pipes, and slide down ropes, in your quest to...well, I've no idea, as developer Fool's Theory hasn't revealed much about the plot.
There's no firm release date for Seven yet, other than what Phil learned last year (publisher IMGN.PRO reckoned then that it would be done by the second half of 2016), but you can find out a wee bit more about the game from its official site.