I find the variety of things to do after you have setup your character/s falls away, though I believe there is enough in the game to easily remedy this.
Allow previously onetime events to be repeated (say once per month or something like that), the reason for time gating is to stop people chain repeating the same location. The goal is to give players a reason to re-explore all areas of Tryia expanding the “end game options”.
Things that could be repeated are; Hearts, Skill Points, Vista’s and Personal Story.
The rewards for subsequent completion would need to be enough to encourage people to re-explore Tyria especially in the lower level areas.
One of the other things I really enjoyed about GW1 was hard mode and timers on missions/dungeons, which could also be used to add more “end game” variety.
For hard mode, instead of buffing monsters why not further down scale the player another few levels. In the open world this could create problems but if the “hit to tag” was significantly increased only for hard mode players, 3rd party healing/buffs were also reduced when in hard mode and other such minor tweaks the hard mode player not will not overly benefit from those playing normal mode in the open world. For instanced areas; like re-doing your personal story and dungeons hard mode is a lot easier because everyone in the party enters at the reduced level.
Timers were also great thing in GW1, not only does it allow time poor players to complete something without fear of it dragging out it, the added benefit is creating variety to existing content. For example put a speed timer on dungeons. If you finish it under the timer you get an additional speed clear reward, if you take too long then your party wipes or they could still complete but just get the normal reward.
This can also open up a whole new range of achievements essentially doubling the size of the game.