With regards to miqo'te, the 26 Seeker tribes we see in Eorzea are those that came here during the Fifth Umbral Era in search of prey. Likewise, the Keeper families seen in game are just the most important and well-known families in Eorzea; the devs have stated directly that other families exist, including some that didn't come to Eorzea until more recently. However, Seeker tribes and Keeper families are below the level of clans (the SE term for subraces), so that's not exactly what you're asking about. 
Could there be other clans? Given how far back the racial histories go, I personally think it's fairly unlikely, but the door hasn't been explicitly closed on it to my knowledge. Given that we know clans can interbreed, it's plausible (to me) that an isolated mixed group could breed and produce their own clan, but it'd be made up of a blend of features from the other two clans of the race with, perhaps, a few novel features. Of course, if you start asserting novel features that you can't actually have in game (such as a lalafell who's seven feet tall or an elezen with slit pupils), you could run into some problems with acceptance OOC.
Could there be other clans? Given how far back the racial histories go, I personally think it's fairly unlikely, but the door hasn't been explicitly closed on it to my knowledge. Given that we know clans can interbreed, it's plausible (to me) that an isolated mixed group could breed and produce their own clan, but it'd be made up of a blend of features from the other two clans of the race with, perhaps, a few novel features. Of course, if you start asserting novel features that you can't actually have in game (such as a lalafell who's seven feet tall or an elezen with slit pupils), you could run into some problems with acceptance OOC.
The Freelance Wizard
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Quality RP at low, low prices!
((about me | about L'yhta Mahre | L'yhta's desk | about Mysterium, the Ivory Tower: a heavy RP society of mages))
