Problem: The existing class combinations in GW cover almost all of your normal fantasy roles.
Problem: When they release the expansion with new classes, you'll probably have to make new characters if you want to use the new classes, unless you only take them as a secondary.
Problem: People keep asking for new classes that are "almost" like an existing class, but with certain key differences that make them thematically different but gameplay-similar.
Possible solution: Instead of adding classes one at a time, add a new set of specialisation classes based on existing ones. Examples at the bottom.
These classes share 1 or 2 attribute lines (plus skills) from their parent class, but have a different Primary Attribute, and different properties (such as armour AL, weapons, energy regen / energy pool, etc.) They have a couple of unique skill lines with their own skills.
For a PvP character, or a new character, it's just a new class that you can choose, just like any other.
For existing PVE characters, in the expansion you'll get quests that allow you to change your primary class to one of the related specialisation classes. Thus you'll be able to keep some of your skills, items, etc, whilst still getting to play the new classes.
Unresolved problem: Can we be both the parent class and the spec class? Eg, be a Warrior/Berserker ? We'd have the situation where we would have less attribute lines than someone else, because those classes share some. If that's ok, no probs. If that's not ok, then we might have to complicate it by preventing such combinations.
Edit: Oh my god, I never realised that Warriors and Ele's have more attributes than other classes! Some people can't count ^^; So problem solved :P
Anyway,
For example, let's say Warrior -> Berserker
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Berserker:
A warrior that wears light armor, but has high health and 1 pip of natural health regen. Has 1 pip of energy regen and 10 max energy, but can gain adren at a higher rate due to Rage, the primary attribute.
Primary Att: Rage (lots of frenzy-type skills, affects adren gain rate, etc.)
Atts:
Axe Mastery (from warrior)
Hammer Mastery (from warrior)
Battle Cries (offensive-related shouts)
Battle Hymns (enchants? Frozen-soil effect? I dunno, just making it up on the fly...

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Then we have Warrior -> Sword Saint (aka Samurai)
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Sword Saint:
A warrior who uses a sword in an exquisite dance of defense and offense. Wears lighter armor (70AL?), but has 4 energy pips and 25 base energy.
Has no attributes related to shields or foci, so you'll have to rely on a secondary to use a left-hand item.
Primary Att: Duelist (improves attack speed, lots of Riposte-like counterattack skills, etc.)
Atts:
Sword Mastery (from warrior)
Blade Arts (spell-like attacks that require a sword. Ranged, aoe, pbaoe, etc.)
Inner focus (self-buffs, "next hit does 1.5x damage" type skills, etc.)
Soft Arts (evades, dodges, movement skills, speed skills)
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Here's some example spec classes, just pulling them from my arse:
Warrior:
Sword Saint
Berserker
Knight (heavy armor, tactics)
Elem:
Summoner (Creates various elemental creatures, pet-attacks, related spells)
Spell Sword (Additional spells that work like Conjures, IW, etc. self-protection skills like Armor spells and wards.)
War Mage (Big fat AOE spells. The nuclear catapult of the fantasy world.)
Ranger:
Archer
Animal Trainer
Druid
etc....