Very much a sandbox. oPvP with a heavy crime system to discourage people from killing/stealing too often. A huge emphasis on being able to mod and create content. If at all mechanically possible being able to drop items in that open world and steal them from other players. Probably freaked half of you out with being able to steal others items, but as a result of pking + stealing you can: Create anti-outlaw factions/bounty hunter/guard factions that protect and retrieve items/throw outlaws in jail.
Being able to create houses, keeps etc. in an actual environment that can be influenced (see ArcheAge). The housing area would probably be in a plane of oblivion so that justification for increasing space for housing isn't lore breaking, though building on someone's plane of oblivion and getting that to fit with the lore would be interesting. No instanced housing, it would actually be apart of the game.
Fully destructible buildings, though this probably wouldn't apply to player made houses. Mostly would apply to PvP environments but some PvE as well. It'd be incorporated heavily in dungeons ex: breaking through a keep full of undead and laying siege while protecting against hordes of undead rather than just killing the undead.
Somewhat similar to Neverwinter, players would be able to create dungeons, and possibly apply their own rules to them (I'm mostly thinking of Foundry from Halo 3).
Trying to keep as close with the TES spells, skills, attributes etc as possible. That would mean levitation, jump, mark/recall spells as well, but it would take up a lot of magicka to channel or cast.
I think I would leave everything else they've done untouched, maybe have it set in another time. Since I'd probably be under the same limitations as ZOS, and this isn't TES6, I would set it in the past, though being able to continue on with the Thalmor conflict would be pretty amazing. Thalmor vs. Empire - letting players dictate the course of the war and how TES 7 would play out as a result of it.
Basically the ultimate sandbox TES style, encouraging any and all kinds of freedom, freedom to kill other players and freedom to hunt those who kill, freedom to mod a little more than just LUA, freedom to build housing in an open world environment, and freedom to create endless content that would keep others occupied.