The 9th profession should be: None. The 8 profession already in game to fill most if not all niches. Just give them more weapons, traits, utility/healing/elite skills, weapon skins, armor skins etc and you can make pretty much anything.
I agree with this. I doubt we'll see a new profession anytime soon. It probably won't come until we go to a different continent and we have so much of the current map/dragons to uncover in Tyria that it'll definitely take awhile before we see new professions.
BUT IF they add a new profession I think it'll be a heavy class. Maybe they'll add two professions like they did for each of the GW1 games? (Factions, Nightfall)
I think they'll make a Heavy ranged class (Paragon-esque) at some point and maybe even consider redoing the Monk completely to be a hand-to-hand specialist.