Ninja was never meant to tank. Shadows were suppose to be used to avoid dmg if they pulled threat. The player base did the unexpected and took a class in a different direction than the developers intended and it worked.

We already have War and Pld for tanks and if they were going to add new classes I would rather see either:
DRK, SAM, THF or BST

Back when PLD was the tanking class in XI I would run PLD and my friend ran RDM and when NIN became popular and I couldn't get a party as PLD we switched roles and I did BRD and he did NIN.