So I've beaten Dawntrail and seen the sights. I have a few thoughts on where things might go from here. Note : this is long and covers patches and expansions that I think may be in the future.
The coming patches will involve the "key" and Alexandria as well as what may likely be referred to as a Vestige of the rejoining. On mention of that, I'm guessing that even when a world is rejoined to the source there are fragments left in the place it was at. Alexandria was on one such fragment on (going by what was shown during the talk in the Crystal Tower) the 12th which succumbed to lightning. During the patches I'm going to assume we'll go there and deal with a powerful entity tied directly to the element of Lightning, similar to the Cloud of Darkness. Since we've already dealt with the Old man back in ARR and Eden, and considering the black metal known as electrope is going to be in that realm, I'm thinking we'll face Ramewl. The rest of the patches maybe more about ensuring the stability of both Alexandria and Tural. The last two set up the next story.
I think the next expansion will involve both the southern Isles and Hingashi. With the "key" available we'll get invited into Hingashi proper by one of the ruling class that wants to change something there. I think they'll have access to another vestige world, either Ice or Fire and want our help with controlling access to it. Whichever one they have access to in Hingashi proper, the other vestige mentioned will be in the South Sea Isles. Similar to Ramewl, I'd guess seeing Ifreeta and Shivalry as the owners of those elemental vestiges.
From there the next expansion could be in either Meyrcidia or the areas around Garlemald. If it's Merycidia, I'm still of the mind that it'll be a return of the Allagans and their cloning tech. Tiamat calling for our aid and we'll find that during the original war, when they cut down Bahamut, they took his eyes, and possibly a clone of him, loyal to whatever leader this Allagan group has, is now fully grown. Honestly I think that Bahamut is in some way stronger than Midgardsormer and this one will be just as strong since he would have Bahamut's eyes.
If it's the areas around what was Garlemald, then I don't really have any ideas of what it could be since I don't think they plan on taking any more inspiration from FF12 or Tactics. I suppose there could be some odd play out of auricite being parasitical beings from another star, since both Sabik and Ultima seem to be such things. In which case they could Introduce a Jenova like creature into the mix or something similar. It could lead to there being some lost city in the northern reaches from the time of the ancients.
Obviously there are also the other living shards, but I think most of those will be many expansions from now. I think the vestiges of old shards will hold some of our future trials. Obviously still plenty of places to go though.