Necromancer is a possibility. There's Eldiem Necropolis, and there's also a NPC that's a necromancer involved in a CoP quest, in Tavnazia I believe. As an example, the Homam/Nashira NPC talks to a Blue Mage in the CS when you trade the pieces in, that were a huge tip-off that Blue Mage would be one of the jobs involved in the new expansion. Hence why I had been talking a long time before it were announced that Blue Mage would be one of the jobs. Additionally, SE did unintentionally mention in the interview that going to the pre-war Tavnazia could be a possibility.
However, Necromancer were one of the more popular jobs that the fanbase at allakhazam wanted, and everyone knows SE tries to please them. SE could not see how necromancer could work in the game, so they gave us PUP instead. Either that, or they saw that as a better chance to explain Alexander

Either way, one job involving death is somewhat likely, considering the circumstances. The second somewhat likely job is Mimic, since it were a rather popular job in past games. However, as of yet there's really no mention of an NPC with mimic-like abilities. However, I'm still crossing my fingers that Mimic will be one of the jobs.
In all seriousness though, in order to figure out what the jobs will be, we'd have to see what NPC's are out there, since those jobs can't have entirely died out

And SE will most likely put hints in the game about the next expansion as it draws nearer. That's what they did with CoP. The entire expansion mentioned Far East and their problems, and then they added a BLU NPC before the job were released. The entire NPC helper storylines were around Near East, and they hinted about Near East in other places as well.
So my theocraftery (is that even a word?) tells me the following:
1) Necromancer (would be the out-of-nowhere job like PUP)
2) Mimic (Would be the "obvious" job?)
3) Insert in-between job that's been in previous game but no one could see how it could work (read: Corsair). So perhaps Geomancer