if the OP means splitting the tanks into two categories of tanking then it technically already exists. In terms of defensive cds WAR/DRK lean heavily towards ehp mitigation i.e. TBN, ToB, Equilibrium etc. PLD/GNB is more standard with flat mitigations. However in EW its become more homogenized so every tank has at least one cd that is either a self heal, shield and/or % mitigation.

Regardless I think they hit the upper limit of jobs because they've shown that they can't add a new job without gutting an existing one and giving those skills to the new job. At this point, once we've hit an even number of jobs in a given role i.e. 4 tanks, 6 melee, 4 phys ranged, 4 magic ranged, 4 healers, they should focus on emphasizing each jobs unique thematic identity and stop adding jobs. Personally, I find it redundant to have multiple jobs that all play the same. TLDR; same shit, different smell.