It is well-known in the Magic rumor community that R&D honcho Mark Rosewater has a so-called shortlist of block concepts to draw upon in design. These are things that are resonant fantasy archetypes, which players would immediately recognize and be comfortable with - like "Ancient Greek world" for Theros or "Gothic horror world" for Innistrad. On his Tumblr blog and elsewhere, Rosewater repeatedly mentions a few of these, leading to the conclusion that they're near the top of that shortlist:
- Pirate world. One periodic topic on Blogatog, Rosewater's Tumblr blog, is this idea of a plane covered in oceans with seagoing variants on typical creature types and naval battles. This so-called "Pirate world" seems to be met with skepticism by the rest of R&D - the consensus seems to be that it is somehow too cheesy to be done well - but really a swashbuckling adventure world isn't more than a hop, skip, or jump away from Zendikar, and heck, it could even take place on a flooded version of Zendikar.
- Ancient Egypt world. Riffs on real-world historical cultures are pretty commonplace in Magic blocks, going back all the way to Ice Age's pseudo-Scandinavia of Terisiare, but one of the most recognizable ancient cultures hasn't even been touched on yet. There are a ton of places this setting could go, even if the naysayers make unflattering comparisons to Yu-Gi-Oh.
- Return to Dominaria. Yes, back where it all began. In not too long it'll be an entire decade since we last visited Dominaria in Time Spiral block, and although that particular block did serve as something of a fitting conclusion to the plane's entire arc, the fact is by now it's probably unfamiliar to a majority of players and besides is huge and diverse - there are still stories to be told about the rebuilding of Dominaria after the time rift crisis.