Aren't sure what Mystary! is? Read up on the game in our first article about it, called Rumours, Speculation and Wild Guesses. I have to say that concept artwork by Chris Glenn is pretty sweet.
So did you read the article, or just saying something and hoping it will be true? It's all speculation at this point, which means we know nothing for sure.
Although post-apoc is certainly possible (the character's dress looks quite Road Warrior), I don't think that it's any more likely than Arcanum II. It has the midieval/modern mix that would make sense for the world of Arcanum years after the first game.
I'd have to put my bets on something with a science-fiction touch. That may possibly include post-apocalyptic. I also have to guess (going from the concept stuff and presuming it's for Mystary!) that it'll use the HL2 engine. Can't see why he'd do a concept of a desert raider and make a skin and 3d model for it if it's not going to be used in a 3d engine of the calibre of HL2's.
Concept art for isometric sprites is often done to look 3d -- partially because the sprites are made in 3d, or at least have to be seen from different angles. I remember the concept art for Torment. It was very similar (not in look but in 3d-ness). That said, considering everything is going 3d, this probably will too. This, of course, pisses me off as I hate 3d for RPGs, but oh, well. It's just that the concept art proves nothing.