Granted I haven't done a lot of tanking, and frankly that is why I am asking for a magic/ranged tank. I don't like melee and I doubt that I'm alone in that if there was such an option I would be more into tanking, and I imagine others as well. So all that said, maybe let's try to think of a way to make this work?
First off, if the pet had basic controls and a decent sized aggro pull it could bring in the mobs just fine. From what I have observed in dungeons this could lead you to using the tank to pull mobs then tell it to move to a certain spot much like a fairy or other summon we have currently. Next would be in dealing with positional mechanics. To be honest...there would be no way to conventionally manage this without a LOT of micro managment. So instead maybe have a mechanic based around shields that the mage bolsters with it's own damage. Without it, the mage has to sacrifice some of their own life to either restore the shields or repair/heal the pet. This also would get around the healer needing to heal both mage and pet, simply have the mage be the only one to heal the pet.
By linking the life of mage and pet you could have a reasonable way to be ranged while tanking, bolstered by having the pet simply attack what the mage attacks much like normal pets do currently. Now yes this would in effect make the pet 'easy mode' for a lot of mechanics, but at the same time the tank has to keep track of the fight, teammates, themselves, mechanics, and their pet's status. So I feel it would balance out.
Well yes and based on what I wrote above it should be. As for aggro there is already abilities to shift aggro around from person to person via the NIN, so having a tank pet just soak up all the aggo the caster produces isn't outside of what can conceivably be put in the game. Furthermore having an aspect of the tank that can't be helped by teammates is also already in the game, unless the Dark Knight actually can get help from AST or BRD jobs with their mana regen?
Granted this would either be considered an easy mode job because the pet can ignore/soak a portion of the mechanics that other tanks can't, or it would be considered the hardest tank due to having to keep track of yourself AND your pet as you tank. Maybe that is a good thing, after all the point is to have a tank that is NOT like the others with a different flavor and weapon. In fact with a high physical defense, magic defense, and high dps style tanks...without going ranged there honestly aren't a whole lot more options to go with.