Maormer are the most reasonable, seeing as how the Wolf Queen made communications with them and even planned an attack with them, they would have to do a lot of svcking up to be excepted in Tamriel though, or maybe like a splinter group from the original kingdom of Maormer looking to take refuge in Tamriel. They should have like a high bonus to Illusion and 1h weapons, seeing as they are war like and their skin is that of a chameleons.
Of course, TES will more than likely never add another playable race with the exception of Orcs and Imperials, although they were already native to Tamriel
Well, Orcs where added even when they got no homeland. And the "real" Khajiit did not come untill Daggerfall. In Arena they where just humans who was big Khajiit-fans.
Imperials came in Morrowind. I say, i's about time they gave us a new race.

Sea Elves are probobly native to Tamriel. And now that the Aldmeri Dominion is back maybe they return and become part of it? But then again, they left after the War of the Iles when the other Elves and the Empire drove them out (thanks to Wolf Queen tricking them in to war with their former allies) and no word has been heard from them for 300-500 years so maybe they are not welcome in Tamriel anymore?
On another note, Tsaesci Snake men are still supose to live in Tamriel.