* Yes, GW2 is probably about a year off, at the very least.
* Don't delete your old assassin! It's probably already earned some pretty valuable birthday minis and will continue to earn more.
--> You didn't really specify if you
only have Factions...?
* You'll need Nightfall to be able to make one of "them new classes." In PvE, Paragons and Dervishes are far from meta and are actually in need of some help (devs keep promising a dervish update, though). I have my doubts they're sought after in PvP, but I wouldn't know for sure.
* If you intend to do any serious PvP, you'll want to have all campaigns so you can have all the skills that are expected of you. Doing this will also allow you to get through the PvE easier, with heroes, so you don't really have to rely on human teammates.
* Adding Eye of the North will give you even more skills, heroes, open up
War in Kryta (if you also have Prophecies), and a
Hall of Monuments that you can start filling to get rewards in GW2.