There is no answer to this question because it's about preference. I have 9 chars all with 5+ heroes. All my chars are lvl 20. All my chars have access to the 3 campaigns and EotN.
I spend most of my time capping elites (mostly for use on my heroes) and getting my monk further in all the games (parties are always in need of monks). But that's just me.
Guild Wars, from that perspective, has an immense amount of replay value. My decisions on "what to do" revolve around which of my friends are online, their agenda, and which charcter/profession I'm in the mood to play.
If you currently have only a few chars, just set some simple goals for them.
And like Onarik Amrak suggested: Just play.
