I just had an epiphany and it may not be accurate but may be the route SE is heading. The current base classes are sufficient with cross class abilities etc. Since the new jobs are 'classless' and for the fact the jobs will require any class to be at level 30. Could it be that the new jobs will have a soul that you equip to any of the existing base classes? So you could have various combinations like you had in FFXI?

For example and this is just for discussion purposes.

Could have a DRK/THM or DRK/MRD and the base class which will determine base class abilities