In terms of a setting, existing possibilities would be the Great Seas and the Emerald Dream, both of which I think could support a full expansion on their own, provided they are fleshed out the way Pandaria was. A 'Dark Below/Underworld' themed expansion would also be super rad, although there's not a lot of existing lore to support it, other than Azjol-Nerub (which was a zone Blizzard was developing but then cut from Wrath).
If we get a Legion expansion next though, it could take place on some Legion infested worlds - maybe modern Outlands will get some expanding with new zones, or the un-used dark portals there can transport us to other worlds. Or just straight up take us to Argus. It could happen.