That'd be a quaility of life change which honestly puts some use to the UI. As of now, mana is entirely useless on MNK, NIN and DRG but is still there because reasons. If each job had it's own interface to show their respective resources (such as ACN having a better indicator of their aetherflows, or the aforementioned BotD timer). It lends a bit more flavor to the job. But I'd imagine they've shot themselves in the foot with the design of the UI, since classes like LNC, PGL and ROG can use mana, but job crystals take away that ability ,but is still technically the same class.
To chime in, this also goes in line with the cross skill mechanic. Honestly it really adds no depth to the individual jobs other than button bloating. It's literally the same skills sans traits and takes away from the individuality of the job. Especially for "essential" abilities like invigorate, swiftcast, and provoke. Instead of giving each job their own way of managing TP, it's all done in the same manner. Tanks lost a flavor in the taunt department, and swiftcast feels obligatory for healers in almost any content. Not to say remove it completely, but those utilties and dps would be baked into their inherent moveset, if not added as a new skill entirely since you're freeing up 5 skills from cross classing.
In general, cross-skilling always comes back to the same problem; there will always be optimal choices and ultimately the ability to select those traits takes away from job identity and homogenizes them. Not to mention something like mechanical traits are designed with the job in mind, whatever works on archer may not nessescarily work on gladiator. For a big change to work, you don't want to take away the idenity of a an existing class, you want to expand on it.
It's both imo. The way the class/jobs are now, you can't reasonably make branching jobs due to how they share abilities and traits from 1-50. The way cross skills work now, they are essentially required (and this really isn't up for debate when a fight demands tank swaps) and you end up having skills that are exactly the same on multiple jobs, taking away from individuality, fantasy and flavor. Jobs not having traits is also locking it to those soul crystals, and leads to button bloating (which cross skilling also does) while being unable to add depth or identity.