I would say that the base classes (Gladiator) branching off into multiple Advanced Jobs (Similar to Skill Tree) seems most likely.
GLD (PLD/DRK)
MRD (WAR/BST)
The reason I think having the Jobs move into Advance Jobs such as the layout in the OP is that endgame will look like this.
SCH x 2 + THF x 2 + DRK + 4 -> Given the idea that all Sub Jobs skills will be available to Advanced Jobs + New Advance Job Skills
That is unless they take it all the way and have an Advanced Job for each pair of Job classes.
(BLM + BRD = Dancer)
(DRG + WHM = Holy Dragoon)
(PGL + WHM = Oracle)
(GLD + BLM = Geomancer)
etc, etc, etc,
Which would be huge. Possibly too huge.
Edited: (Didn't have my morning coffee yet)
Either way would be good. Jobs moving to advanced jobs 3 into 1 would eventually provide more than 3 new Advanced Jobs anyways.
Example
•GLA/PLD-------\
•ARC/BRD-------->--->Enhancer
•MSK/?RDM-----/
•PGL/MNK------\
•CNJ/WHM------->---->Blue Mage
•ACN/SMN---------/
•THM/BLM------\
•LNC/DRG------->---->Templar
•MRD/WAR------/