The Thirteenth is definitely a potential future candidate for something, optional quests in Shadowbringers made that pretty clear. Really feels like they built that up to leave that as a potential option in case it's something they want to explore. Not sure whether it'll be an expansion, raid, Eureka or something else though. I can see the Reaper job delving into that though.
Out of the 13 shards (plus the source), there has been 7 Rejoinings, putting us at 8/14, so there's only 6 worlds left. Excluding World of Darkness that puts us at 5 worlds total. There's definitely a possibility that future expansions will take place in one of those worlds.
We know the next expansion will take place at least in part on the third unexplored continent (Ilsabard) where The Garlean Empire is, as well as Old Sharlayan and Thavnair. My guess is that the expansion will start off with taking us to Old Sharlayan, and will then take us to Rads-at-Han on Thavnair and have us travel north towards The Garlean Empire.
The third continent is huge though, larger than Aldenard and Othard, so they could potentially add more zones there in future expansions.
Also, let's not forget that the dragons are literal aliens who travelled to this world, there's any number of potential things they could do in future expansions with that.