Considering the trouble they had balancing Summoner and Scholar while they were attached to each other, I really do not see it happening at all, especially since they have said they will never do it again and the way the 2 jobs are handled now, it wouldn't surprise me if they were fully split next expansion.
What I believe most do not realise is that by adding another job to a class, that job is stuck with that classes abilities to work with. Taking your example with Gladiator. DRK would be stuck with all the Gladiator abilities, including Fight or Flight, Iron Will and Circle of Scorn. Add to this that any adjustment to any Gladiator ability would also affect both PLD and DRK. Just by changing one thing it could throw the balance out. This is essentially why SMN and SCH were so hard to balance. A change in a DoT or ruin affected both, which often made them see saw between which one was too strong. This is ultimately why the current iteration of SCH literally only takes Resurrection from Arcanist, everything else in the SCH kit is classed as a SCH ability, even if it has the same name as the SMN equivalent.
This also doesn't address the issue of different weapons for different jobs and how the animations play out. If you notice, the same ability is used the same regardless of weapon, every tank uses Interject/Low Blow/Reprisal etc. the exact same way. The same is true of Scholar and Summoner, they use spells in the same way. So, even if you go the route of, you equip a DRK Job Stone, you can use great swords, how would the animation for something like Fast Blade look when used with a great sword. It probably wouldn't look good at all. Not to mention that weapons are tied to classes and not jobs, hence Summoner and Scholar using the same weapon.
You could say, well, when you equip the DRK Job Stone, you update the animations. At this point, you might as well make a whole new job and not tie it down with the added restrictions imposed upon it by having it be tied to a class that already has another job attached to it. Bear in mind, since you used 2 tanks as an example, I used 2 tanks. The situation gets weird when you try and make a Healer/DPS or Healer/Tank. their class kit will be based mostly on healing, leaving no room for a comprehensive dps/tanking style to fit without completely reworking all the level up abilities for the class anyway.
All in all, I really do not see how it could be beneficial to have 2 jobs off of 1 class. They tried it, realised how much of a nightmare it was to balance, and have all but scrapped it since. This same logic could in theory give an insight into why skill trees are hard to balance, but that is a different challenge altogether.
Don't worry I feel the pain with monk. It was the second job I levelled after Paladin and after Heavensward it has been a struggle to continue to use it. Which is a shame, because I enjoy the freeform combos and positionals, that's why I tried it in the first place, but with essentially no evolution through the expansions...yeah.