I'm not sure if I like this, as I've currently got a lot of thoughts about this in my head right now, so I'm going to try and make this as coherent as I can.
I'm worried that any class/job that fills a "hybrid" role is going to cause the same problem as what happened in WoW with it's classes, specifically power creep. The cost of hybridization, the downside to the jack-of-all-trades, is that while they can fill multiple roles, they are inferior in those roles compared to the people who specialize (willingly or otherwise). In WoW, the Rogue, Mage, and Warlock were the kings of DPS because that was all they were capable of doing. The classes that could DPS and some other second role (Warrior, Priest, and Shaman), didn't have viable damage, thus they could only reliably perform their second role. Paladins, who could do all three roles, sucked at tanking and damage dealing, and where thus shoved into being healers (and even then, I don't think they performed well).
That wasn't to last, though, and eventually the classes that could fill more than one role began to catch up to the others. Rogues, Mages, and Warlocks weren't king of DPS anymore, so people were less inclined to bring them along. After all, why bring a Mage when you can bring a Shadow Priest, who could deal damage just as good, but could also heal for you if you needed a healer? When I left, there was still discord among the Rogue, Mage, and Warlock community because the desire to have those classes in parties was still pretty low because they couldn't diversify.
The point I'm trying to make is that, is it possible that any introduced "hybrids" could cause the same problem? That their lack of power gets them buffed later on, but the buffs are so good that people stop wanting to bring the specialized jobs? Why bring the Paladin if the Mystic Knight can tank just as well, but also go DD? Now, I know people can switch jobs in a heartbeat, that the Paladin could easily have a DRG secondary job that he could DD with, but why level two jobs when leveling one nets you the same two roles in half the time?
In addition, how viable would a "hybrid" role be in terms of abilities and gear? If you have a job that can tank and heal, do you have to carry around multiple sets of gear so you can fill both roles, or do you only need one set, but it has so many stats it needs to hit that it runs it's budget out too quickly?
How many abilities and traits do you need in order to be successful at your role? Sure, Scholar comes off a DD class, but with the exception of those DoTs and the INT boosting traits, everything else is still viably useful to a healer. Would these hybrid jobs have to come off specialized classes that are build from the ground-up to let it comfortably fit all its roles?