Now that they've shown that they're okay with introducing jobs without base classes, anything is on the table. I'm personally fond of the idea that future jobs will all be "advanced jobs" a la Final Fantasy XI, and I suspect that samurai, red mage, and blue mage are in the pipes.
That said, what I would really love are new advanced jobs to correspond with the base classes, which the dev team had previously said they'd do. You could really create drastic differences. Marauder --> Beastmaster, Pugilist --> Puppetmaster, Rogue --> Dancer, and Archer --> Ranger have my vote, for starters. What, though, would lancer do?