It has been over three and a half years since Guild Wars was released and personally I still enjoy playing it. When ArenaNet released the 'Eye of the North' expansion (almost a year ago) they said it would be the last piece of new content for GW before the release of Guild Wars 2. I have to admit that I was a little disappointed at first, but I was also excited at the prospect that GW2 was in the works. Now I'm craving a new campaign to GW because I know that GW2 is still a long way off. I definitely think there would be a market for a new campaign, even with the advent of WAR and WotLK only months away. Since GW uses a pay upfront pricing model, it has an advantage over the subscription type MMOs, because you can keep going back to it over and over without having to re-sign up. This is ideal for the casual gamer.
I don't think there would be a problem finding new class types for a new GW campaign as the forums have flooded with ideas for years. My two favorite choices being the Druid and the Weapon Master. The Druid would be a shape changer with healing and beast skills (similar to a Ranger). The Weapon Master would be a melee class capable using using any weapon, and would specialize in turning melee attacks back on the attacker. Awesome!
ArenaNet made a mistake when they took development effort off GW and moved it all to GW2. They should have expanded their staff, not moved it off one project and on to another. There was still 2 years worth of new GW campaigns and expansions that they could use as a cash cow.
Hopefully ArenaNet will hear my plea, and begin work on a new campaign right away, so that I can have something to play over the Christmas holidays ;)