I've read lots of threads on the idea of new classes and I think there are generally some great ideas out there.
However, in Guildwars at least... lets look at what a class really means...
1) The model
2) The type of armor you can wear
3) The kinds of skills you can use
4) The playstyle focus of that character.
so..
1) The model
This personally doesn't bother me all that much. Sure more models would be great, but it doesn't really affect gameplay.
2) The type of armor you can wear
This one is important, but only so much as in the role your class is expected to operate in i.e. Toe to Toe or at range. Oh and of course look and feel
3) The kinds of skills you can use
Ok - now *this* is really what it's all about. The skills a character can use really defines what it can and can't do. We already have an attribute system to limit the effectivness of skills which aren't specalised in.
4) The playstyle of the character
Warriors are designed to go toe to toe. They're built for it from the ground up and thus their skills, attributes and armor are all designed around this fact. You could *try* and play an E/W, or M/W but most people would agree it's working against the design of the game.
So really, if we wanted a new play machanic, which is what we're really looking for... there's nothing stopping the addition of an additional attribute, say Brawling and creating a set of armor/skills which maximise the effect of those skills.
You could even go as far as to place restrictions on the skills you can use with a given armor. Basically you can't use a lot of the best brawler skills without wearing brawler armor.
Atempting to add an entire new class to the game really ends up feeling like double up to me. We already have so many of the basic roles filled within the game:
Melee/Tank = Warrior
Healer/Protector/Buffer = Monk
Damage/AoE/Support/Distruption/Spiker = Elementalist
Damage/Support/Debuff = Necromancer
Generalist/Snipper/Trapper/Distrupter = Ranger
Disrupter/Anti-Magic = Mesmer
If we can figure out a role which isn't already being covered, (which can't be achieved by adding additional skills/armor types to an existing class) then a totally new class would be great. Otherwise, I think we'll see new skills, armor and attributes being added to existing classes to enchance the gameplay options. It's going to be easier to add a handfull of new skills to existing classes than it is to add a whole set new of new ones to be balanced across *all* the classes.