With the launch of Stormblood

[Reynhart]

Personally I don't really mind having subclassification into different kinds of tanks

Problem is, since every content can be accessed by a random party setup, you can't design content where one tank job can't MT. So, if all tanks can be good MT, all tanks should be good OT too.

Right now we have 3 tanks and 3 combinations of tanks, if we have 4 tanks we'd have 6 combinations, if we have 5 tanks we'd have 10 combinations, and so on.

Well, right now, we have 3 tanks, and 2 clearly superior combinations. Create another OT (Still viable as an MT), and you'll probably end with only 1 superior combination.

but that'll be a lot harder to balance when we get more and more new jobs

Frankly, I really don't look forward to a 4th tank...In fact, I wonder how many tanks other MMO managed to do without clear overlapping or serious balance issue.