Elder Scrolls lore has a lot of unsolved mysteries.
Are the Akaviri snakes, men or both?
Is Trinimac really Malacath?
Are ALMSIVI actually dead?
What happened to the Dwemer?
There are theories, opinions and points of view on these questions and others like them but no real definitive answers and to be honest I kind of like it that way. The unknown and unknowable adds to the universe in my opinion and to just get a straight up "yes or no" answer would, in my opinion, make it boring. So what Elder Scrolls mysteries would you like to remain a secret?
Well, only the Tsacei, a race on Akavir are said to be snake like. As for the ALMSIVI, from Morrowind, we at least know Sotha Sil, and likely Almalexia are dead. Vivec could be around still, maybe he is dead, maybe he just up and left one day. Still I would like to see the Dwemer return, maybe as a main threat.