While I know you didn't mean "ranged tank", Nektulos (I merely responded to the "how would such a thing work" comment, incidentally giving an example of such a thing already existing in another game), I'm slightly worried regarding the whole 'casting while taking damage' thing.

If you make "cannot be interrupted" part of their tank stance, wouldn't that take them one step toward being over-powered? I haven't played PLD much at all, so I haven't seen at what rate they get interrupted while casting Stoneskin or mini-Cures (don't know if their new, innate healing abilities have cast bars and as such would be interruptable as well). And yes, I know your suggested RDM wouldn't have access to healing -- which in itself seems like a miss, since afaik having access to both attack magic and healing magic was part of the Red Mage setup -- but I'd like to know your thoughts around this "cannot be interrupted" thing =)

Also, if 3/4 of their aggro skills (just skimmed through your proposed list, might have missed something) are their physical attacks, my concern is that there is very little time/incentive to actually use their magic, other than when off-tanking. Since holding aggro is a tank's job, elaborate side-rotations (that take longer than physical skills due to cast bar) aren't really that feasible. If you look at the current tanks, their non-aggro skills are either the "off-tank combo", or off-GCD instant skills that you throw in while waiting for GCD to tick around. Again, I'm not saying this to immediately reject your idea, but more to understand how you're thinking, maybe even help you develop it further
^^
(And yes, I know a good tank doesn't need to focus on their 1-2-3 aggro combo all the time, and can even do stance dancing in order to do more damage, but aggro combo is still the basic foundation of a tank.)