I imaging most have these have been thought of already but I'll be honest that I finally beat the main story with my AD toon and now am thrust into grinding the EP quest lines. My biggest gripe with it is that I have a lvl 31 NB in EP already so it's going to be a LONG repetitive grind for a lot of wasted hours. Not really into the whole PvE thing but I think I might just go over to it for now until new endgame content comes out.
My ideas for different endgame options would be:
1). Staying with your faction where you spent all those hours finally winning the war against Molag Bal and being able to play PvE campaigns against the other factions still.
2). To elaborate on the first one, maybe playing some kind of assassination missions where you can still play the other factions quest but with a twist so yours will end up winning the quest by killing a key NPC or sabotaging the mission. That way a ton of content could still be used with tweaks for your faction.
3). Maybe making some of the Cyrodill quest tie in with the main story, as in having some of the key NPCs of your faction in Cyrodill fighting the war and giving you different solo quest but being able to tie in rewards or standing for your faction.
4). Maybe adding a new zone, to start, where you can work with other factions players to fight against a different Daedric prince and his battle to take over Nirn. I know it sound like Molag Bal again but at least it would be new quest geared to VRs and allowing players from other factions to interact and group without trying to kill each other all the time.
Just a few ideas and would love to hear if anyone else has any other ideas.