Right now 1-3-2 is being discussed, and in that regard the off-tank role will be destroyed. I don’t like the idea of getting rid of the off-tanks. They are usually the most fun of the tanks to play, and are a great support for the main tank. I predict an abysmal time for a single main tank.
Instead I propose going for 2-3-2 - simply take the current setup, but add an extra DPS.
The game already puts heavy importance on a single character and at higher levels, a fight is often lost if a single hero dies. With 7 heroes instead of 6, each hero will carry less weight, meaning it will be easier to make a comeback after losing an ally. It also means there are more targets, and of course also more CC, more ults etc.
To balance it out, there may have to be changes to the maps. Maybe more space needs to be available, maybe more flanking routes etc.
Another thing one could do to adapt to this situation, is to reduce the amount of especially CC. Maybe CC abilities could be on a slightly longer cooldown (maybe 10% longer on average). Also ultimates could be both toned down and made rarer by making them take longer to earn.
This would have the overall effect of desaturating the game of effects to “make room” for the extra hero on each side.
IMO instead of getting rid of the off-tank role, it would be way better to explore a 7-team setup.