Or will characters be so customizable that clear cut classes won't really exist?

Guess:

Warrior- Two Hand Build- AoE DPS, Dual Wield, Single Target DPS, Sword and Board, Tanking
Rogue- Archer- Ranged DPS, Thief- Sneaky backstabber, Bard- Support/Healer
Mage- Destruction- Ranged DPS, Restoration- Healing, Conjuration- Summoner DPS

Based on information, it seems to be class based. These are my guesses on the classes.