The Final Fantasy franchise has always had a strange relationship with direct sequels and multiple universes. Final Fantasy 10 and 13 both received sequels, among others, with the latter also gaining a threequel. Final Fantasy 15 received a ton of DLC to fill in the holes of its story before their cancellation, while the universe around Final Fantasy 7 is large.
Now, Square Enix is focused on expanding that universe even further with Final Fantasy 7 Remake, and Final Fantasy 16 is also in development. That's good news for fans of the franchise, as the next few years since filled with content, but it's possible that more will come along too. Indeed, based on recent comments, Final Fantasy 10 getting its own threequel is not out of the question.
Final Fantasy 10 character designer/developer Tetsuya Nomura and designer/writer Motomu Toriyama recently discussed the possibility of Final Fantasy 10-3 in an interview with Famitsu, which was then picked up by Japanese sites and western media. According to Nomura, the chances of Final Fantasy 10-3 happening is "not zero." He'd go on to say that, "If we are to make 10-3, we somehow have a synopsis written by Nojima-san...It's sleeping right now, but the plot itself does exist."
In other words, the story and outline for the game does exist in an official capacity, but it is not in any sort of active development. Much like an outline exists for The Last of Us 3 but nothing is proceeding with it, fans are kept waiting to see what actually comes of it, if anything. The possibility of another entry in the subfranchise is likely exciting for many fans of Tidus and Yuna's story. It doesn't really end there though, as not only the possibility of it happening was discussed, but on the off-chance that it did, there was even a small time table established. As Toriyama explained, "we are not in a phase where we can talk about it, unless we have finished producing FF7 Remake.”
As such, if Final Fantasy 10-3 were to happen, it would be several years away. Final Fantasy 7 Remake is an ambitious project, with all eyes on multiple events and characters as Final Fantasy 7 Remake Part 2 amps up, but Part 2 probably won't be the last part. This means that it's years away from being done, and that the slim chance of Final Fantasy 10-3 happening is even further away.
As aforementioned, the franchise has always had a strange relationship with the notion of direct sequels. Getting another Final Fantasy 10 game more than 20 years after its last sequel would perhaps be the strangest, but even among titans like Final Fantasy 7 Remake and new upstarts like Final Fantasy 16, it's impossible to say it wouldn't be welcomed.
Final Fantasy 10 Remaster is available on PC, PS4, Switch, and Xbox.