This is my opinion of the changes to tanks shown during the NA fanfest and the criticism i have of what has been shown so far.
This is meant to simply act as a letter to the development team and not a discussion on the changes.

The Main Tank / Off Tank split
Splitting the tanks up into these two defined role is going to create alot of negative friction at all levels of play.
Before going into specifics, the way that people play MMO's always works on a trickle down effect from the high end players to the more casual players. This means what is the most optimal plays at the top levels try to be emulated by everyone all the way down to the bottom level. While it will likely be fine to clear all content with any comp as we have always seen throughout the games life that does not mean that certain jobs will not be forced into specific roles. We already see this in the party finder in extremes where the standard comp everyone does is 2 tanks, a pure healer, a barrier healer, 2 melee, 1 phys range and 1 caster and 1 player per job. When pretty much every bit of content in the game is clearable with any comp for the most part.

With that in mind, I will go into the specific problems that will be caused by this change.
- The split between what can MT and OT is skewered based on if your playing Reborn or Evolved. Example: The new tank can only play as a main tank in evolved mode, whereas every other job can do both in both
- This takes away alot of freedom in how you can play the game now for example you might be forced to OT as a warrior because your co-tank is playing the new job and he can only MT
- This also creates alot of friction with the duty finder, If you have the new tank and a Paladin playing in evolved, both will want to be MT and this can cause them to fight over the spot because they want to make full use of their kit, you would not even be able to prevent this situation like you could using the party finder.
Solutions
- Give the new Tank a reborn mode, this will allow them to be able to fill any role.
- Adjust the split in tanks to not be MT and OT, simply be based on the type of raid wide mitigation they have as they currently do e.g. PLD+WAR both create a shield on the party, DRK+GNB both reduce damage taken. This could be designated as "Pure Tank" and "Barrier Tank" similar to the way we have healers.
- The "counter" skill that PLD now has which lets them do an attack in response to taking damage is a cool idea however make sure it can proc off of intervention, that way a MT can still do the same attacks and mitigation regardless if they are MT or OT.

Provoke and Shirk and tank stance changes in evolved mode.
Provoke and shirk are now on the same action and change depending on who your targeting and provoke lets you generate more aggro for a period of time, this later buff replaces the current tank stance.

This causes a whole number of issues across the game and future design of encounters.
Example:
Oracle of light, She needs both tanks to be first and second in aggro for the akh morns and the auto attacks.
With this change, it means that Tank A has to provoke to get aggro and also needs to position the boss, Tank B also has to get aggro asap as well, which means using provoke to start generating aggro which means they are now positioning the boss. That means that Tank A has to provoke again to get aggro again. Now they have no provoke stacks for if a tank swap comes up quickly so they cant provoke and shirk.

- The changes give less freedom in recovering from mistakes
- They also make positioning a boss where both tanks must be first and second in aggro such as the Tyrant even more incredibly annoying.
- Lower geared tanks maybe have issues holding aggro against more geared tanks requiring them to use provoke to hold aggro meaning they may not have it when needed.

Solution
- Keep the current way the tank stance works, a simple toggle on/off as its own action
- With the tank stance staying the same, the provoke and shirk change could stay the same.

Speculation:
This is simple questions and problems that would be cause dependent on how things could go.
- MT shortage, at the start of Evercold, there will be a flux of MT players with the new tank job releasing, however as time goes on and into future expansions these players will drift away from that new job, you will have the issue that you will be waiting a long time to find a MT due to only two jobs being allowed to do so
- As OT's will be focused on lowering damage taken by allies and dealt by the boss, this translates to raid wide mitigation and debuffs on the enemy (the latter being something we used to have on all the tanks combos_. The question becomes, can the debuffs stack on the enemys from each OT, if the answer is yes then that means the OT'S will be way more valuable than the MT's in hgiher end content. That would mean that in order to have the MT's have a place, there must be tank busters only they can take (based on them being skilled at taking direct damage) This would mean that alot of the teamwork of optimizing fights will have been removed as with certain attacks only being able to be resolved by a single member, everything will simply fall into place.

Extra
I apologise for some of the long winded sentences and any spelling errors.
I have been playing the game Since 2.0 and tanking for that long, I really enjoy tanking and being a MT in particular, I do not want to see any of the tank jobs forced into a specific role as it would shrink the number of jobs i would have available to play on at both a high level (HC, Ultimate, W1 e.t.c.) and at a casual level (Extremes, exploration zones e.t.c) as a MT role.
I am happy to see the changes to the combat system and the jobs to receive changes that highlight their uniqueness but I do not wish it to be at the cost of their ability to engage with the game at all levels.