Please do not lock Jobs to the Weapon Class that unlocks them.

[Cairdeas]

Gladiator can be easily shifted to two roles: attacker and tank.
Marauder can also have a tanking role and an attacking role.
Pugilist could be attacker, debuffer or evasion tank.
Lancer is debuffer/attacker.
Archer could get an attacking role and a debuffing role.
Thaumaturge and Conjurer can be specialized to nukes/debuffs and healers/buffs

Weapons define available roles. A Marauder tank does not need to be a Paladin to have the same available role, but giving an archer a tanking role makes no sense. So while role does not define a weapon, weapon does to some extent define a role.

Why does using an Archer as a Tank make no sense? A Tank is someone who can keep hate, prevent the monster from moving, and allow the other party members to maintain an easy attack formation.

If an archer can keep hate, stay out of range and keep the monster bound, Are they not tanking it?
I'd rather see unique new approaches to tasks. I know FFXIV is not the place to try and enforce these ideas. But I'd love to see a game where you picked a Weapon and a Role. the Role determined what you did in combat, and the Weapon determined how you performed it.

IE.

Pick a Great Axe/Healer. You act as a front line healer. Dealing damage with your great axe, earning the power to cast curative magics by dealing that damage.

Pick a Bow/Tank you act as a ranged tanker. Able to to hold a enemies hate and stay out of range by pinning them to the floor with traps or abilities that restrict movement.

Or Fist/Elemental where you cast magic into your fists and punch the magic into a monster.

Thats how FFXIV sounded when they first announced. Sadly We'll never see a game like this I think, and thats disappointing.