Balancing MT vs OT is terrible design.
Especially when you think about what "OT's" do the majority of the time (Which is DPS and Shirk the MT). Essentially meaning that the epitome of OT is... Ninja who can DPS, toss their enmity onto the MT and can even quell classes like BRD/MCH initial burst enmity (Which they can't do because Refresh/Tactician only cuts enmity rather than reducing their generation)
Also, nothing stopped them balancing the current tanks as "MT" and "OT" with DRK being either a second MT or second OT outside the fact that they just didn't really decide what to do with DRK.
To say nothing about how hard it will be for them to balance the 2 jobs in the role, let alone the 2 roles.
Since I guarantee you that the meta will never be 1 "MT" and 1 "OT" job and will end up being either MT + MT or OT + OT due to how the role works and how much of their Tank design goes out of the window because of the way damage is dealt by bosses (I.e. Basically just Tankbusters and Raidbusters that are noteworthy. Which leads to this "MT" situation where people just don't tank swap because why bother when you can just cycle things like Cover and Intervention to never have to)
To say nothing about how this will screw over people who like a particular job, only to get shoehorned into a particular role late game because instead of getting some reasonable balance *Cough*PLD's Snap Enmity*Cough* it's decided that they're going to be inferior to a "MT" and just be a support character instead.
If they just balanced Tanks properly, so that all tanks could MT effectively and all tanks had some form of "OT" utility that would allow them to be an asset to the party in situations where there's just 1 boss that needs to only attack 1 target at that period of time. That would be perfect. Since then all tanks can be used in all roles.
This means, not having inequality between defensive skills (Such as Holm being a shorter CD). Not having a single job having a monopoly on additional CD's that can affect a MT (Such as Cover and Intervention that allows a MT to effectively have 2-3 additional CD's). As well as not having a disparity between overall DPS output (Otherwise you'd end up just stacking the 2 highest DPS Tanks because DPS > All)
Tanks should have a strict baseline shared throughout the role. This is the one time where some level of homogenization is useful. Mitigation skills for Tankbusters should be equal across the board, irregardless of which 2 of the 4 Tanks you have in your party. Just like how healing requirements should be met irregardless of which 2 healers you run. So, not having PLD bring 2--3 extra CD's while OTing, not having WAR with 25%-60% more "Invulnerability" skill usages (Though, I'd argue that having these types of skills usable against Tankbusters is always going to end up with poor balance unless they're all carbon copies like Role Actions...).
These actions can have unique flavour for each class, but the core should be that when dealing with TB's, each tank set up should be on equal footing. If not, then it lends itself to compositions that have the more tools to negate TB's (Such as a reason why WAR + PLD is so popular)
The rest of Tanks kits can be flexible though. Make their DPS rotations completely unique, make their utility skills unique. Maybe even give them unique "OT" skills (Though, you'd need to be careful to not create any DPS increases such as how Shields build more LB). Give them "Active Mitigation" that's unique so that they function differently in between TB's (If this damage ever becomes relevant)
But the important thing, is to make the core function of Tanks equal. Otherwise you will never achieve a balance, even with "MT" vs "OT" designs (Because if OT's are still capable of doing stuff like 4 man content, that means they have some personal CD's... If they're then also "OT" designed because they can use CD's onto a MT... Then OT + OT parties will have more CD's available to deal with TB's as both tanks can swap and use these extra CD's on the active tank)