Morrowind implies (directly states?) that there were once many more Ashlander tribes aside from the four that exist in 3E427 (the number sixteen sticks out to me but I may be mistaken). Does anyone know where I might find information on these tribes and if any of them might have ceased to exist as recently as within ten or so years of 3E427?

Sadly no, most ashlander history is unknown to us probably due to the Ashlanders distrusting the houses, temple and the outlanders. Ashlanders don't write things down, they remember the honored dead with prayer and word of mouth from the wise women and the elders.

I also think the act of tribes dominating others or tribes fusing into one through marriage and diplomacy happened.

Is there anything else you remember about that statement about there being more Ashlanders? Personally I don't remember anything like that. For a tribe to disappear as recently as ten years ago, I assume that would either be down to the blight or colonists whether Imperial or House Dunmer.

The only thing I remember specifically is that there were more tribes in the past than there are now, though I couldn't give an exact citation for that. It may be from a book or general rumors that pass through the game...Like I said, that there were once sixteen tribes seems like something I read somewhere once, but I can't find a source on it now so my mind might be playing tricks on me. If there's not really any info on the subject, I suppose that gives me some wiggle room.

I imagine it would have been disseminated by now if it were in a book. The searches I've done through the Morrowind game dialogue have turned up nothing, including for "sixteen". If you could remember even a small sequence of words in the dialogue, that might be helpful, but I researched the Ashlanders pretty heavily a couple years ago and didn't see anything about more tribes. Though nothing that would contest the existence of more tribes, either.

Sorry, I can't remember any exact words. My last playthrough was about a year ago and, being roughly my gazillionth, I didn't exactly pay attention to all the rumors or secrets (I suspect it came from a "little rumor" or "little secret," or else from the conversation with Hassour Zainsubanni).

Oh, wait! I just had a memory. I believe my memory of additional tribes may come from a Temple quest in which you are asked to http://www.uesp.net/wiki/Morrowind:Cure_the_Outcast_Outlander. It's also possible I'm think of (outcast) http://www.uesp.net/wiki/Morrowind:Camps not tribes.

(For the curious, I'm working on a mod that would add a character who is the last survivor of a lost Ashlander tribe. I'd like it to be as lore-friendly as possible, but what I'm getting here is that I have quite a bit of leeway. Might still take the tribe-name from one of the camps, though.)