There's not much ways to tank. PLD and WAR are both based on high enmity modifier and large effective hit points. WAR was even patched to have better damage mitigation, so it's closer now to PLD than it was before.
Thereforce, future tanks will surely have similarities too, wether it's "creative" or not. Their unicity is not based on theses mechanics.

No, it's not enough for PLD to be a "super GLA". They could have just made GLA like PLD. The purpose of "specialised build" if the choice of several builds. Like SCH and SMN are both specialised Arcanists.

Arm of the Destroyer doesn't prevent MNK to be a good DPS. The fact that it can Silence is a nice addition, not something you have to use all the time, thus crippling your DPS. Every job is not "just" its role. PLD has a DPS stance, WAR has DPS buffs, MNK has a healing buff, etc...that's what makes them unique in a party.