Regarding weapons, I'm 95% sure the game must have each weapon be main class + optional job. (To allow unequipping job crystals)

So if for example Conjurer would go RDM with swords, it would mean Conjurer could equip swords too. Which is unlikely.

So my own speculations would be things like Gladiator - RDM, Pugilist - PUP, Marauder - DRK, and so on.

The remaining issue from this, is how some jobs would not offer much variety to the existing one. (Unless say DRK would be a debuffer more than a DD)