1. "Adventurer" is one profession among many.
This means the aspiring blacksmiths, thieves, bankers, bards, shopkeeps, etc. a. have fun things to do without killing anything, b. can progress tradeskills entirely through the things in part a
(Hiring adventurers to bring back crafting ingredients is, however, encouraged. Ideally, take any NPC quest giver and imagine how to put a player in that role.)
2. Allegiances are not all set at birth.
This means all races can guild together and group together outside Cyrodiil pvp. There should also be smaller pvp conflicts where players can choose sides based on conscience more than race.
(Example: an isolated fort that can be claimed on behalf of orcs or necromancers. Any race can pick either side. Winner affects nearby quests and spawns for X hours.)
3. Home is where the heart is. (And gameplay is where the home is.)
Players should be able to claim somewhere in the world as home, and the game should respond to this. This is not about hearthstones or owning buildings - NPCs in one's hometown should respond differently.
You'll have to roam far and wide to really progress, but your chosen home should never become obsolete or irrelevant to you regardless of level. Radiant-like quests should crop up that aren't available to people just passing through.
4. Interaction is more important than convenience.
If someone in Elsweyr needs ebony bars, they should talk to a player recently come from Morrowind and haggle on a price - not pick an anonymous lot someone listed in Ebonheart that gets mailed automatically. Quests should come in all geographic scales - some across town, some across aetherius. Sometimes, you should need help from players outside your area of expertise - a lock picked, a scroll deciphered, a letter of introduction from someone better acquainted with the nobility.
5. Nobody's story is the same.
Suppose there's some cave in Skyrim full of mooks. Alopho the Altmer and Nils the Nord team up to take it on. Nils doesn't have Alopho's objective, because its a procedural step between two handcrafted segments in another province. (Alisa the Altmer got a different cave.) Alopho doesn't have Nils objective, because although Nils is experiencing a handcrafted story specific to the cave, the quest giver visits people at random. Procedural hooks instigate handcrafted adventures and vice versa.