I just finished the last main quest in the game using my sorcerer. I think I have maybe a half dozen zones where I have not finished all of the side quests. When I go do that, until Gold Road comes out, I will no longer be able to do questing on my sorcerer. Unless I do daily quests, which I will likely do long enough to get all of the motifs unlocked. I have even done most if not all of the quests for the dungeons. I have collected all of the sets for all of the zones. I have not collected all of the sets for the dungeons. I have barely touched trials.
I have purchased all of the DLC content. I do run some dungeons, but I really enjoy questing, which means that even if I were to cancel my ESO plus, I could still play all of the content.
I still do not know what I would consider endgame.
I think when I am done on my sorcerer, I am going to start doing the same thing with each of my other characters. My warden is the next closest. The other characters are going to involve a lot of work. I think that if ESO gave me the ability to set the difficulty where I wanted, while questing, I would be set for life on this game.
Technically that is not fair. ESO is the only game I play, and that will not likely change until Elder Scrolls 6 comes out.
What do you consider endgame content?
For me, I rather enjoy the process of questing and running around Tamriel.