The class least likely to be added?

[Nalien]

Onion Knight could be a Limited Job like Blue Mage, just rather than learning spells from monsters, you learn skill by leveling all the other Jobs in the game.

Mime, similarly, could be a Limited Job. Depends how they're going to work future ones (whether they have the same learning gimmick or not), but all it needs is a "Copy" skill that you use on party members to copy their last skill. I do doubt they'd ever bother with that though, Limited Jobs are meant to be fairly OP and non-party focused, if Masked Carnival is anything to go off, not sure how a Mime version of that would work... Hope we see Gogo some day in XIV though...

Honestly any Job could simply be a Limited Job at this point, but I'd have to say Beserker is probably the least likely to ever be added. A Job which sole function was that it automatically attacked and did nothing else? Yeah that wont even be considered for a MMO, even if it could technically be a workable Limited Job... Much more likely they just rolled it into Warrior at this point. Similar deal with something like Time Mage, time magic would be hard (though not impossible) to implement in a MMO, and once upon a time I'd say Astrologian had a few core Time Mage aspects, but those were gutted recently...