Why though? What's intrinsically imbalanced about them?
Warrior/Combat
Warrior is, in my humble opinion, the most lacking of the 3:
1) Lack of skills that deal with obstacles like locks, etc. That means using a magic item (not necessarily bad) or leveling a minor skill (bad times).
2) Weapon wise, a player will typically use only one type. Bethesda has made the same mistake in skills (One Handed and Two Handed bloat the Warrior skillset)
Overall, the warrior skillset has been gimp, inflexible and repetitive in 2 games thus far. Will the trend persist in Skyrim?
Mage/Magic
The Mage is quite a contrast to the Warrior:
1) Tons of versatility. Can sneak around as well as a Thief, and endure as many hits as the Warrior with defensive spells.
Can do virtually everything that the other archetypes can do and more. There will be incentive enough to play a pure mage in Skyrim, perhaps too much.
Thief/Stealth
The Thief, overall, is actually closer to balance in my opinion. Moving Alchemy to Stealth was a good step in balancing the sets, but here's some possible future issues:
1) Speechcraft and Mercantile bloat the skill set. It makes sense to combine them
2) This is speculation, but Thief melee potential, either in sneaking or in the open, has to be observed. A thief shouldn't hit as hard as a Warrior, but a Thief should have a CHANCE of winning if exposed, even if it is slim. Stealth may not always be an option, and who would use a dagger against a Dragon?!
From what has been shown in interviews, dagger kills are really leaning on sneak attacks (10x damage modifier!), and this could be problematic.
Conclusion
As I've said, Classes may not exist in Skyrim, but that doesn't change the fact that people will still decide to focus on pure builds. The skills from each Specialization need to ensure that each archetype can stand on it's own while still retaining weaknesses and unique playstyles.
Thoughts?
So on that note, here are my balancing suggestions for the classes. Tell me what you guys think about them.
Warrior
-Massive amounts of health and incredible melee damage
-"Realistic armor" i.e. wearing a heavy armor made of huge plates of metal covering every inch of your body should give the specialized warrior incredible endurance to attacks
-The highest stamina of all the archetypes
-The ability to bash locks and intimidate enemies and merchants alike <perfect for perks
Mage
-Huge hit to magickal efficiency when wearing any armor
-Lowest stamina of all the archetypes (they should have to stay and fight with their powers without the ability to run away easily - no cowards among mages, I say)
-The specialized mage should keep the ability to do what other archetypes can do, using magic, but at the cost of bonuses like Sneak, Alchemy, Heavy/Light Armor, Speechcraft, and all other combat skills
Thief
-High sneak attack bonuses, with sneak kill moves
-Mastery of the bow
-Middle amount of stamina between the warrior and the mage, to allow fluid combat movement and evasion, but to a limited extent
-The specialized Thief has the unique ability to get past locked things quietly, charm merchants and possible foes, and to create potions/poisons that give him boosts in combat or in general
-The specialized Thief should also not be able to survive in open combat for long, and must rely on escape, potions, and ranged weaponry to survive
*Keep in mind everything I've proposed about the balancing of the archetypes is solely based around the skills each archetype will include in it's constellation in TES V*