Blu and rdm would still be able to tank better than PLD.
You could easily find ways to stop both of those jobs tanking(although i see nothing wrong with having 3-4 tanks as opposed to everything can tank, as it is), ie. give mobs conal tp moves or auras that have horribly dibilitating unremovable debuffs (slow2, silence, massive mp draining disease etc) which they could then make pld and nin immune to via new JA. Ultimately Square would use their own ideas anyway so spending too much thought or arguing specific ideas as a player is pointless, they were just to show a few ideas of things that could be done to show it's not impossible, they could make up proper fixes (being paid to do this stuff and all).

When I mention this stuff of course I mean new proper raid mobs, HNM or bigger events. Not abyssea fluff which is now finished and done.

Personally I would like to see blood tanking back, fix defence etc but they probably won't do that either.