There are many races we haven't seen even though they are described in the lore. For example, I'd love to see some Lilmothiit in Black Marsh with quests to kill them/drive them out. In my opinion, I think it would be interesting if they were a sub race of Khajiit.

There are other races such as Sload or Maomer, which would make a great addition. Don't get me wrong, I don't want to see these anywhere I go. I'd just find it interesting if we could find one or two NPC's at the Summerset isles that were Maomer, or a secret cave on an island outside Hammerfell which was inhabited by a group of Sloads. There are many creatures in the Elder Scrolls lore which are extinct - they would fit perfectly in this game, and one of the reasons they were extinct could later be added because [insert Elder Scrolls Online reference/event here].

Neither Sload nor Maormer are extinct, and it was confirmed in an interview between Paul Sage and TIL (or was it Matt Firor? I never know the difference) that Moarmer are in the game, presumably PvE enemies in the AD.

The Lilmothiit, which I would love to see, are sadly most likely to be extinct (UESP says by the time of the Akaviri Potentate, which is way before TESO). Of course there could be some random tribe still in existence, but I doubt it. It would be cool to see their ruins in Blackmarsh, though. Like catacombs full of zombified fox people.

Way before the Akaviri?

I do believe that they are still around in the 2nd ERA!