It's criminal that this game has so little in the ways of pvp with such an amazing combat system. WvW is a great concept, but it really really needs improvement.
EotM and Obsidian sanctum both have an arena clearly made for fights, but since you are hard-bound to one of three teams it makes fighting a friend on the same server a chore especially now that you can't really have an accurate fight since pvp/wvw/pve have separate skill stats. An FFA area / mode would be nice. Or, you know, implementing dueling.
Open world pvp (a server dedicated to it) would be a godsend to me and would change the whole dynamics of the game. Or better yet, become pk flaggable like mmos of old. Sitting around as a non-pk player and watching two player-killers in open world suddenly get in a fight was always entertaining.
I believe the only way to solve zerging / blobs would be to implement player collision so things like choke points would actually be choke points. And no, a player doesn't have to be a immoveable object. There are many effective ways this could be implemented, for example; being able to slowly push people around.