If precedent is followed and the canonical Nerevarine was a Dunmer, they could well still be running around Akavir.
Why he would have to be a Dunmer to be still alive? Am I missing something?
It mostly follows from two things. First assuming that any potential game in which we could meet the Nerevarine occurs after the events of Skyrim, its been long enough that, barring special snowflake status, a Nerevarine who was a member of one of the races of Man, Orc or a Beastfolk would have died from age, so a living Nerevarine would need to be one of the Mer.
The second, and somewhat flimsier reason is that, the only example we have of meeting a previous player character is the Champion of Cyrodiil/Sheogorath, who appeared as an Imperial Male in Skyrim. With Oblivion having been focused on the Imperial province, this then would suggest that the champion of the game that focused on the Morrowind province would be a Dunmer. Obviously with Sheogorath being himself it's entirely possible that he has changed his appearance since his days running around in Cyrodil, but it's the only datapoint we have to go on.
Now it's possible that they would go with the Nerevarine having perished long ago, in which case they could have been of any race, but the idea here is thinking of what would be necessary for meeting a living Nerevarine.