I believe this kind of thing is only possible if Naoki decides to revamp the job/class system a whole lot since many jobs are pretty heavily rooted in their role (tank, healer, dps).
CNJ, for example, has these traits;
Freecure - Grants a 15% chance that after casting Cure, the MP cost for your next Cure II will be 0.
Overcure - Grants a 15% chance that after casting Cure II, your next Cure III will cost half MP.
As well as these skills;
Cure, Cure II, Medica, Medica II, and Esuna
With GEO having 4 skills that are basically worthless, and 1 godlike skill for a DPS, I fully believe that it would be imperative to rework CNJ heavily.
As for GLA traits;
Enhanced Rampart: Increases damage reduction granted by Rampart to 20%.
Enhanced Convalescence: Improves the HP recovery increase granted by Convalesence to 30%.
Enhanced Shield Bash: Extends Stun duration inflicted by Shield Bash to 6 seconds.
Enhanced Awareness: Extends Awareness duration to 25 seconds.
Enhanced Sentinel: Increases damage reduction granted by Sentinel to 40%.
All these traits are basically useless to a DPS but sticks pretty true to a tank focused job. Now their actions you've touched upon but that still leaves out abilities like; Shield Swipe, only useful while tanking and blocking; Bulwark, useless if not tanking; and Provoke, which, unless you're trolling heavily, is not something a DPS should ever use.
I'd like to see classes get existing second jobs but the way they have the classes set right now, if it wasn't made for two jobs to start with in mind, it's glaringly obvious a massive rework of all the classes' base abilities and traits has to be done to accommodate for a second job of a different role. Then there's advocates trying to get a second job of classes that are the same role. However, that's a can of worms for another thread.