This thread will assume a few things.
-You kind of know about this particular area of the lore.
-You consider yourself a lorebuff
-You understand what happened to these races as far as has been thus revealed, and their interactions with other races. At least to some degree.
So what this thread is actually about then?
How would/could WE, the lore community, rationalize certain races appearing/coming back into the game.
Such as...
Kothringi returning in some fashion. The Crimson Ship sailing away and having survivors.
Lilmothiit having remnants of them on a far off island south of Black Marsh.
Left Handed Elves having remnants who lived through the Yoku induced atomos split and their genocidal tendencies.
Non corrupted Snow elves returning in force. Guess we could assume some kind of reversal of the Dwemer's corruption processes.
Non-devolved Dreugh. Apparently the Dunmer devolved the rulers of the previous Kalpa by hunting them for their materials(Dreugh Armor).
Bird people that Topal the Pilot mentions on his journeys up the Niben from the Topal sea..(Might be fanciful fiction-within-fiction on his part)
Etc Etc...probably missed a few.