'Endgame' in a MMO should be entirely relative to the person playing, rather than a set point in a linear path to be fair.
For example if I want to be a raider, 'endgame' to me could be setting myself the goal to amasses a certain amount of wealth, or have a raiding party of xxx people.
If I want to be a member of the BoS, 'endgame' to me could be setting myself the goal of working up the ranks to be granted the honour of more exclusively technology or access to more information in the achieves.
The modern expectation of all games (or more particularly all MMOs) to have defined paths where everyone gets to the same target and then the game presents pre-created scenarios and situations using developer made content & goals is something that's always niffed me since 'Raiding' and the sort became the demanded norm.
Have such content for those that wish to do such activities certainly... but don't package such things as 'endgame'.. provide the tools and the options leave it up to the player themselves to set their own endgame objectives in mind...
It's really the only sane way to make a MMO that isn't going to cause the developers to constantly be chasing themselves to release the next set of 'endgame' prefabricated content.
Edit: I know the OP doesn't actually present 'endgame' as a linear path nor do they mention 'raiding' or similar, but simply posing the question of 'endgame content' really makes the distinction that some content will be purely for when you reach the end of the game... which is what my rant is regarding.