Throwing in a new weapon or two (since not every profession would use the same weapon) is a decent way to introduce new skills and matching traits to all of the existing professions at once.
Adding a new profession is neat, but obviously it makes the existing professions old, so hopefully any new profession they add is built around a bunch of new weapon types everyone else can use. Thus, even though there's a hot new profession in town, everybody gets something cool and new.
I'd rather see them add one new profession, a Heavy armor class to round out the numbers; there's currently three light professions and three medium, but Warrior and Guardian are the only heavies. I'm slightly OCD about the balance.
Anyways, we've had this discussion many times before, but let's have it again;
Great Axe/Two-Handed Axe: Warrior, Necromancer, Ranger (maybe Guardian)
Fist Weapon: Warrior, Guardian, Thief, Engineer, Elementalist, Mesmer.
Whip (1h): Thief, Ranger, Mesmer, Necromancer.
Glaive (1h Spear): Warrior, Guardian, Ranger, Thief, Mesmer
Halberd (2h Spear): Warrior, Ranger, Elementalist, Necromancer
Javelin (Off-hand Spear): Warrior, Ranger, Thief, Necromancer
Give Thief staff (for some bo-staff support-style ninja/monk-ery) and Engineer hammer, maybe (since it seems like a useful tool for a combat engineer).
Also, Engineer needs a new kit, a steampunk-looking multi-gun that has all sorts of bits and pieces from other firearms slapped onto it haphazardly to give it a lot of versatility. Give it a Sniper Shot (Increased damage the further you are from target, for a long-range profession like engineer, this seems like a no-brainer), an underslung Grenade Launcher (no manual targeting, has a slight AOE but causes bleeding to one target), Rapid/Automatic Fire (like the warrior Volley, just fire off a ton of rounds quickly) and a special explosive round you can fire and then trigger to detonate at a specific range (like the Guardian orb). Maybe that one does fire damage.
The 5 ability would be the best though; you manually target a spot on the ground and throw the weapon to that location, unequipping the kit. If an ally picks it up, they get to use the weapon for a while (like an elementalist's conjured weapons), but if an enemy gets near it, it triggers a booby trap that causes a ton of conditions. Like the Dragon assault rifle from Perfect Dark, basically.
Give it some badass name like "The Mauler", "Southsun Special" or something and we're good to go.
Anyone want to toss out a third Heavy armor profession?