ArenaNet’s Martin Kerstein has passed us information from Eric Flannum, Lead Designer for Guild Wars 2, about the criteria for new races and professions:

There are a lot of decisions that we have to make when we add new races and professions to the game. The evaluation process goes something like this for races:

• Is the race visually and culturally interesting (are they cool enough to be a playable race)?
• Do they provide us with a different sort of archetype that is currently not in the game?
• How difficult would it be to implement the new race from an art/animation standpoint?
• Can we write a compelling story for them? What is the lore impact on making them playable?
• Does the new race provide us with the opportunity to make cool skills and abilities?

For professions, the evaluation process would be very similar but in a slightly different order of importance:

• Does it provide us with a different sort of archetype that is currently not in the game?
• Does the profession provide us with the opportunity to make cool skills and abilities?
• Will those skills and abilities work well with existing professions, or will they unbalance things?
• How difficult would it be to implement from an art/animation standpoint?
• How do they fit into the lore of the game?

With professions, we’re much more concerned with how they interact with the current balance and whether they provide a different play experience. With races, their overall look and feel is generally the most important thing.Of course, this is because while race does have some impact on how a character performs in combat, it’s nowhere near as important as profession in that regard. Likewise, a character’s personal story is impacted greatly by race and only very minimally by profession.

Source: Room for Future Races and Classes? at GuildWars2Guru.