The danger with branching out a lot from a defined role is that they become a closer form of a hybrid class/job. In games that only feature the holy trinity as their party form (with XIV being more of a focus), this breaks their balancing and often even makes them OP compared to other jobs (regardless of role). It can even start to make that job perform better than jobs of other roles when combined with the other abilities that define it. Lots of complaints inbound with that.

I'd absolutely love to see a return to some imbalance, akin to old school MMORPGs, since that often makes jobs extremely fun to take part in playing. Tanking Nidhogg/Fafnir/etc as a BRD in XI prior to the Mazurka nerf, for example, was pretty damn fun since it was a change of pace from the norm. BRD was arguably a better tank than all the other jobs for some endgame content, despite it being a support role. Likewise in XI, the discovery of players making NIN a tank from its original DPS (DD) design literally changed the games combat design for a very long time. Hell, even in this game, if they didn't nerf Titan Egi from that Ramuh fight so quickly, we might have actually started seeing SMN become a semi-tank for raids/dungeons as a norm.

As such, we can't really do that here, simply because of the fact that balance and role definition is a big deal to the devs (even though some points are arguably contradictory for them doing that). They want to lock the jobs into their established roles as much as possible without branching out. Maybe they'll just follow WoWs tank path with Legion, in tanks healing themselves for significant amounts during a fight is part of the role. It'd help establish the DPSing while healer mindset that is rampant here, at least.