Falinesti.

Since there's no gaps in the Bosmer lands to be filled in, any Bosmer-themed expansion will have to look elsewhere - and Falinesti, being famously missing, is a thing that needs to be explored somehow. Since it exists again, as of the time of TES 1: Arena - whether or not that's the real thing returned from wherever it went to, or a New Falinesti built because the Falinesti Faithful finally learned why they had to give up on the old one - either one leaves space in the lore for finding out what truly happened to it, and either bringing it back or confirming its permanent loss.

Also: Central Skyrim - Whiterun, Dawnstar and Winterhold - the Winterhold mages' college could be the focus of events there, even if they start affecting things as far south as the still relatively civilized Whiterun, whose Jarl we have already met in Greymoor on a diplomatic mission.

There's a big gap in the map still in the middle of Black Marsh.

And to the west of Cyrodiil, perhaps the Colovian Highlands and Skingrad provide a link to what drove the success of Varen's campaign to seize the Empire. (And, potentially, a link into southern Hammerfell, which also doesn't exist yet - and a land route to the Gold Coast, which already has an unusable gate leading "to the Colovian Highlands", also potentiall connecting to Bangkorai and Reapers March.)

The Telvanni, who refused to join in the Pact, must be up to something in their own heartland of Telvannis, east of Vvardenfell. Who knows, they might even have a wizardly rivalry with Winterhold?

It would be cool to have some real pirate city like from thieves guild, but more devastated, free and evil, with interesting characters like from Stitches, without the usual dalies, but rather imbeded NPCs - like someone who does like dogs would teach you how to breed them (similar to Warframe breeding), it would add gambling as a minigame (similar to scrying), arena fights, and it would be conntected to maomer race, which could be added as playable. There could be some really cool main thief and you could do jobs for him or for someone who opose him.
Or I would like to see expansion with hunters which would teach you hunter skilline, which I miss really in ESO, with tracking mechanic the same like those already in the game, animal like companions (now possible with companions added into the game) and trapping skills similar to vampire hunter like those in fighters guilds.

New race preferably. New weapon skill line would be nice too, either technician style or spell weapon type. There aren't enough Magicka weapon skill lines.
Definitely don't want a mini map, I hate screen clutter, if they make one it should be optional.