So after the letters from the producers there is a goal to have each class have two Jobs and also the possibility for Jobs to require having multiple classes to level 50. Here is what I would expect to see and why.

Gladiator is going to get the Job of Redmage as a DPS and have a BLM/WHM requirement.

Marauder will get Dark Knight class and have a BLM requirement. They could bring in a Viking as well for a DPS class.

Pugilist would get a NIN and could be a tank, but most likely a DPS. Could also be a Thief but would guess based on weapon choice it being a NIN.

Archer would get Theif and would give them a knife as a off hand weapon. Another one to keep with the them of the world would be to be a DPS Job as a Gunner.

Lancer would probably be the hardest to have another class that uses lances. I would guess a tank class, but would guess as a DPS as something like a Beast master.