I had this idea earlier, though I'm not sure if it could be a thing. Among the races we already have playable, what if the next ES game, whether this be a DLC or not, we were able to be a Snow Elf or a Dwemer? I'm well aware not many are alive, if none at all, but in Skyrim, should you have Dawnguard, you meet Knight-Paladin Gelebor and Arch-Curate Vyrthur, the two possible remaining Snow Elves. Upon talking to Knight-Paladin Gelebor after defeating Vyrthur, you have the option to ask him things about the Snow Elves, the Chantry, etc. At some point he says it's possible he is the last Snow Elf. In the next ES game, if you chose to be one or the other, you could wake up, not in a prison like normal, but from an eternal slumber due to magic (if not magic, a tonic maybe? or perhaps something similar to the Miasma in Vaermina's Tower?) and the first quest upon choosing that race would be to find out where everyone has gone. You search and find no one, so the main quest of those two races would be to find out what happened, the truth of the fall of the Dwemer/Snow Elves. Of course, even without choosing those races, you could still search for the truth. In Morrowind, you do meet the supposedly last remaining Dwemer who says he could be the last, but he's not for certain.
Another thing was underwater ruins, somewhat like the ruin in the Knights of the Nine add-on to Oblivion where you had to go underwater to find a certain ruin. There could be more ruins, more valuable treasure, more water monsters instead of slaughterfish, maybe even an easter egg or such (not sure of what).
One last thing would be where you start upon the race you choose. For example, if you were to choose High Elf, you'd play in Summerset Isles and slowly progress to the other countries/continents. Personally I'd love to have all kinds of quests to do in each country/continent, or even owning land in them!