To tell the truth, the only jobs that are really feasible and unique enough to be their own jobs in the future are:

Samurai (I agree it would probably be DPS)
Red Mage (It would probably rely on fencing combos and use elemental stances)
Dancer (Basically a Melee Bard that has to keep using their Dance effects to boost the party in a similar manner to NIN's ninjutsu)
Alchemist (Least likely of the group, but would probably be a healer class)
Sage (because CNJ dps job)

It's possible that they would also make a Beastmaster/Blue Mage type class but it would probably lay claim to neither of those classes' name sakes. It would probably be a more 'feral' monk. Every other job listed either already has an existing class set (such as the Onion Knight set/helm) or already exists in another form (Thief/Assassin = Ninja). There's also some that overlap such as Runic Knight and Red Mage, where Red mage is the more popular and more distinctive job.