Start with the current base game (races and classes and traits and skills).
The world is also very nice.
Max level 20. Have the starting zones be level 1 - 15. Have a couple 15 - 20 zones. Then the rest of the world is level 20.
Have more workable builds for each class. As-is, you can look at a character and his weapons and have a very good idea of what he can do. There should be more options available that actually work.
Maybe have more skill options or trait choices available, or have them easily and cheaply switched around.
Make more pvp options. WvWvW is nice, but I spend FAR more time building castles than I do fighting. It really does need revamped. Make the maps smaller, which would force more fighting to happen.
More team pvp matches. Really, more pvp in general. I don't pvp constantly, but this game is very limited in comparison to gw1.
Have lots of cool cosmetics available as rewards for completing level 20 quests. Maybe other things, as well. That way people would have a real reason to keep adventuring.
I don't know anything about the dungeon system. Not sure if it needs anything or not.
Oh yeah, have REAL TUTORIALS. Explain the basics of the game. Don't just drop a new player into a small instance and let him drown. Especially for after you get crafting. Have a small quest that explains it and shows how it works (especially that you can swap professions and not lose any progress).