Could dual-wielding be the key to hybrid classes ?
We've seen at Gamescom that right-hand gives some abilities and left-hand gives other abilities (shield giving Provoke and defensive skills, sword giving weapon skills).
And it seems pretty safe to assume that most armors will be wearable by any class. They stated everywhere that you would switch your weapon after a fight to heal yourself. That wouldn't be effective if you had to switch to mage gear at the same time, dropping your hp and starting with no mp since you had gladiator armors before ...
Example: If you dual-wield a katana and a dagger, that could turn you into some type of Assassin class (ninja and thief mix).
The only hybrids that would cause problems would be FFXI Paladins and Red Mage types (assuming classes like Dark Knights can access both weapon skills and some black magic from the scythes). A Paladin wearing a white magic wand and a shield would not be really **** ... It would require magic swords for these combos to work.
Holy Sword + Shield = Paladin
Holy Sword + Dark Sword = Red Mage
If we keep trying to think outside the box, there could left-hand tools that allows more unusual classes. Examples: dices for gambler, a monster skills memory receptacle for blue mage, a monster communicator for beast-master that unlocks pet commands abilities, etc ...
Combine these tools with another one-handed weapon in your other hand + all kind of different armors and you've got yourself a lot of freedom.
I just really hope they allow some ways to customize your class to be "hybrid" in-battle and not just switch to staff after every fight to heal yourself. That could get boring very quickly ...