Samurai could fit the tank role. As could ninja but I don't think an evasion tank would work that well in FFXIV (without being massively overpowered). "Evasion" is dodging red and other colored indicators. Ninja is still a mandatory job for the game though, it just doesn't necessarily have to be a tank (players made ninja into a tank in FFXI, it wasn't meant to be one in concept while Samurai, which became DD, was supposed to be a tank in concept).

The only issue is lore...and we'll probably need to wait for an expansion before we see samurai or ninja. The most we can hope for is Musketeer, which should've been in the game at 1.0's launch but is still missing.

Archer needs Ranger or Hunter (and Bard needs to be altered from being a DPS job to a support job)
Gladiator needs a DPS job (Duelist? Rune Fencer? Fencer?).
Marauder needs a DPS job (Dark Knight? Viking? Berserker?).
Conjurer could use a DPS job (Sage? Devout?).
Pugilist could use a tank role (Black Belt?).
Lancer could have a tank role as well i suppose...though it's kind of a contradiction to what Lancer's are supposed to be (Sentinel?).

Add Musketeer to Limsa (Corsair-style support and Gunner dps), add a Rogue class to Ul'dah (Thief support role/Dancer healer) or make Alchemist a unique DoH that has a combat job crystal (healer role), add a Red Mage class to Gridania (Time/Green Mage/Geomancer support role and Mystic Knight/Blue Mage dps role).