I vote for some Closure on the whole AD thing, or at least the Forsworn. I'd set it in Valenwood/Elswere/The Black Marsh, because these haven't been done yet, and combined, they'd be large enough to compete with the likes of Skyrim, and Cyrodiil. Probably a War between the Bosmeri, who secceeded from the Thalmor, but were attacked by the Argonians, sensing weakness. Fresh from being kicked back out of Vvardenfell by the returning Dunmeri, they'd be fighting on two fronts as well. Basically a moshpit of the Meric races all fighting each other for Dominion. Lots of Magic, Poison, Disease, and arrows flying everywhere, with the Melee leg of the Combat tripod nerfed for once.
So, the Bosmeri get Archery, and Poison against the Argonians. They see the return of the Shadowscales with Plague Magic, and the Thalmor are weakened by a backfire of their meddling in Skyrim. (The Noodle Incident being the return of Talos to set them straight.) This caused the near collapse of the Empire, who's too weak to get involved in this CF.
Races: (Since this is the Elven Wars of Succession, they use the ...Meri names.)
Altmeri: Pretty much unchanged. Pure Mages, Highborn, nuff said.
Argos get Immunity to Disease, and claw damage that lowers Health, and Stamina until Cured. (Plus a whole spellbook of Plague Magic only availible to the Shadowscale Faction.) Retain Waterbreathing constant effect. Some way of explaining how they took the scorched earth of Morrowind from the Dunmer!?
Bosmeri: Poison Immunity, and probably a daily power which Fortifies Speed 30% for 60 sec. Remove Pick Locks/Pockets in favor of Forestry Skills. (These are Not city Elves.)
Bretons get 1 Handed, and Archery (from their allies, the Redguard, but we're a long way away from there) instead of Speech, and Alchemy. Largely taken over by the Reachmen, and Hagravens, all of this is in the background, but the difference is still visible.
Cyrodians: Rather than the Imperial Luck, probably Resist Shock 25%. (I don't know why this was never given to Any race in the history of Nirn.) Very low presence, the Empire has all but fallen.
Dunmeri: Largely unchanged, basically the Nightblade Class. Absorb Fire (Element specific Spell Absorbtion) Daily Power.
Kajiit: Permanent Night Vision, and Speed Boost. No resistance, nor Claw Damage.
Nordmen: Extremely scarce, as is Cold damage. Swap Speech for Archery, add Thu'um as their special ability. (Frost Breath once per day, slows with a chance to Paralyze. Actually say "Iiz SlenNus!")
Orismer: Some nerfs to make them them not the most overpowered race in the game. Probably 50% weakness to Magic in keeping with the Pure Melee archetype. (Doubled in Berserker rage!) Honestly, with the history of Tamriel, I have no idea why they haven't taken it over, or at least been the Big bad to be beaten back by the Empire at least once. (I could write that too, Orisinium vs the AD.)
Redguard: Adrenaline Rush doubles Regeneration, Magicka, and Stamina recovery for 60 seconds. Start with Bound Sword, but lose their Poison Resistance. Completely embrace their role as Spellswords, and are now interbreeding/cross training with the Bretons.
Hybrids: Reachmen, Nord/Imperial, Bos/Dunmeri (Some of the refugees fled to Valenwood) Altmer and beast races stay Pureblood. Possibly varients availible in Character Creation like Shadowscale (Mage Lizards) Saberwere(Shape shifters, like Rakshasa) and Warmere(Altmer trained in weapons/armor as Battlemages.)
Pregnancy, and child-rearing, including possible interbreeding. (All off camera, go to bed with your spouse, and fade to black.) Some races are incompatible, Men with Men, Mer with mer, the Bretons being the only ones who can cross with both. (They are, after all, Manmers.) Both Beastraces can only inbreed, and the Altmeri generally keep their bloodlines Pure. (They can, but just choose not to interbreed with lower Meri, especially Orismer, "Ewwe!")
Some Allusions to the Dark Times (Dawnguard) Return of Ysmir, and Dragon War. (Back to back Time Crisese, I didn't make up!) Attenuated time-scale, so incubation can take place within the approximated Years of gameplay. Race Specific Questlines with 3 different start points. (Valenwood, Elswere, and TBM.) Lampshade the moratorium on Teleportation. (Because of the Oblivion Crisis, it's no longer stable. Write a book, which can be read for this knowledge.)
I could write all of this...