San Diego Comic-Con is a couple of weeks away, and the Magic panel (featuring Mark Rosewater and Doug Beyer, among others) is supposed to give us some big revelations on Khans of Tarkir and the upcoming block. Of course, there's also a chance that other products will be talked about as well - and Mark Rosewater just answered a question on his Tumblr blog that could point at either Tarkir or a new Planechase release.
When mylifeisafirehazard asked, "Mark, were some of the planes from the planes cards in Planechase reworked into future sets such as Theros or even Tarkir?" he responded:
Ask me this again after the San Diego Comic-Con panel.
Mark doesn't foreshadow things in vain: This means that whatever the panelists are planning on previewing is relevant to whether the answer to this is yes or no. This could be interpreted in one of two ways. First, there's the idea that Mongseng - the plane of Kharasha Foothills - was reworked in concept into Tarkir. They both appear to be Mongolia-like steppe worlds of constant war, given what little we know about them. This, of course, doesn't mean that Mongseng stops existing now that Tarkir is a thing; the Multiverse is infinite and two similar planes can absolutely coexist - witness Mark Rosewater's repeated assurances that a future Japanese mythology top-down block would and could take place on a non-Kamigawa plane.
Then there is the notion that the Planechase planes will be revisited as-is - in either a future Planechase product or another supplement. Planechase 2012 appears to have sold well (perhaps not for the intended reasons, because a lot of Commander players wanted the exclusive legendary creatures), and there is definitely sufficient interest in the Magic community for a third supplement of this type.