Something that's keeping me from coming back and playing alternate servers is AFAIK the lack of account wide achievements.
In my opinion the game is so vast, that per character achievements more or less force you to play a single character, and stay on your server if you are the sort of person that likes to track those accomplishments... but more crucially for returning players like me, a source of motivation, zones to explorer, quests to complete, dungeons I have missed etc.
A ton of content that earns achievements is doable even solo these days with a Mercenary optionally. A lot of older content also is easy due to stat inf;ation etc which is NOT a bad thing since you can have fun just completing a lot of older stuff without struggling unnecessarily.
Say I start a new on a new "oldschool" server. If I had account wide achievements maybe I can check out a lot of the Freeport low level stuff I never did because I usually start in Qeynos. I play on a new server, but I continue to track my accomplishments and exploration of the contnet.
But as it stands, to me starting on a new serrver or even a new character, is like resetting everything. I look at all the reset achievements and it bugs the completionist in me.
If you had account wide achievements you could also have some kind of public rankings, that would be fun for returning players and players of all skills, filtered by dungeons or just exploration etc.
You could also unlock some goodies at different intervals just like Guild Wars 2 does, and that also would incentivize playing on new servers - feeling like you are still progressing regardless which character or server you play.