I've been thinking about this for a while and I think that, when they add new classes, they'll definitely have to do *something*. In some cases, it just seems weird to leave the current subclasses as they are (i.e. if you had MSK available, why the hell would you give BRD LNC instead of MSK), and it would provide some level of impetus for the player base to try out the new class(es) for reasons other than novelty.
Beyond simply adding new subclasses like you suggest (i.e. BLM, SMN, and BRD would likely get MSK added; WAR gets LNC or whatever new tank class is added; GLA gets LNC, PGL or whatever new tank class is added; PGL and LNC probably get ARC, for Raging and Quelling; and WHM and SCH would probably get access to whatever new healer they elect to add, since there isn't much of real use that they could get from any other class), another option would be to could swap out an existing "subclass" for certain classes and provide a new one that is actually useful: if they add THF, get rid of MRD for DRG and MNK and swap it out for THF, which would, presumably, have more useful abilities for DPS than what MRD provides (its really just Fracture and it's only *really* useful for MNK because Fracture's 220 potency actually stands out; I also expect that THF would probably have some kind of enmity drop, which MNK doesn't have and is sometimes sorely in need of); BRD might get LNC replaced with MSK (the only useful thing that BRD is going to get out of LNC come 2.1 is BfB and it makes some sense for the two rDPS classes to share cross class skills since every *other* class within the same role shares cross class); BLM, SMN, WHM, and SCH are probably pretty safe since their cross-class suite is pretty well comprised of numerous useful options, though I could see BLM losing ACN for MSK, possibly, to restrict access to Virus/EfE (which are flipping *amazing*).