With One Tamriel, what is the end game now? It used to be Craglorn and vet dungeons, I guess it is still, because Craglorn is still with group delves left and vet dungeons were made stronger, but I guess there is desire for 500cps to have content on their level/equipment, not only dungeons (it will be always only a few of them) otherwise they will try to push ordinary zones to higher difficulty, or they will get bored.
So what is your dream end game content?
I had 2 ideas:
1. Old idea, but still it looks even better with one Tamriel - Dark side of one Tamriel - the all zones redone with dark quests, which you will get after the main quest, where you would decide to side Molag Bal/Menimarco. But also it would be harder - lets say 200+cps. Or simply previous golden Cadwell Tamriel difficulty, but a new content.
2. an area of the world, where low level players should be killed upon entering, where only a "chosen one"

could enter. The only area I can imagine are Oblivion pockets.
But it would be also an old idea but in a new coat - Craglorn tried to be such idea and none was there anyway.
So it would have to be done differently. I was thinking about making it solo, but scale it from 160cps higher, or make it with survival mechanics, where if you are weaker, you would have the possibility to sneak around and the goal would be some mechanics, not to kill the biggest boss, only those, who would dare to challenge it would have that optional choice to kill a boss.
Such area, that would be really cool to explore would be some nasty parts of Coldharbor or something like that, with maybe some time limit for entering - like you couldn't breath there, or something like that.
On the other hand what if most of the high level players are ok with daily bosses tasks and have no desire for end game? If Craglorn was empty, maybe really it wouldn't be worthy of that effort, only if it would be possible to do it solo, scale it again atleast for level 160 solo and higher?