Well, if you've:-
- completed all three campaigns (missions/bonuses) in NM & HM
- completed all higher end PvE content (UW, FoW, DoA, the Deep, Urgoz) NM & HM
- completed all EotN content (missions/dugeons) in NM & HM
- completed WiK, HotN, and WoC
- completed Sorrow's Furnace
- completed the Titan's quests
- done any PvP you might be interested in
- have all the armour styles you like
... and if you're not interested in chasing after any
titles say for your HoM or just for the hell of it, then perhaps you could roll a new profession and start over, maybe in a different campaign.
I'm sure I've left some things out on the above list, but it's all I can think of right now - and the HM (Hard Mode) is of course optional. As others have said, set yourself some goals and go for it, will help to keep you focussed.
Of course if you're not having fun doing any of the above, then might be time for a break.
Cheers.