The answer:
Non-combat roles and social activities surrounding TES lore.
TES is the most lore-rich CRPGs I've ever played. Even as early as Arena they had fleshed out background on holidays and festivals, which if brought into TESO effectively, would add regular social events to the calendar. Social gatherings like festivals with their own "traditional" costumes (player-craftable) that allow the player to unlock a single-use buff for the completion of any social activities would give players of all types a reason to attend the festivals.
Non-combat classes such as crafters would serve as excellent support classes. Whether it's cooking or armorsmithing, being able to make things for fellow players to use can go a long way towards extending the life of one's subscription to an MMO.
And for those who aren't into social events or non-combat roles, As long as there are plenty of randomly generated quests to go on that do not require a group with the ability to find useful max-level items, it will be well.
Also, some sort of user generated content mechanic would keep things entertaining in terms of story...
The most attractive MMOs are the ones that offer stuff for all playstyles, not just one or two. So an endgame that caters only to raiders and PvPers and leaves out soloists, social players and crafters will not be appealing to me. Others will have playstyles that they would be just as disappointed if left out of endgame.