The more I think about it, the less I want RDM anywhere near a tank. The reason is two fold; BLU would make a vastly better tank, and RDM and DNC have the potential to do a bit of mixing up in the healer/support area's.
Right now Support is done by Ranged DPS, which is great and all, but it would be nice to have a support that get's into the fight. If we put RDM in there with it's rapier and spells, it would be very much continue the mold but with it's own unique twist. But if we put DNC into the melee support role, suddenly MNK has a class that can benefit from it's Dragon Kick as I would assume fan's would be blunt damage.
But there's one type of healing yet to be represented, and that's one done by damage. In WoW (which is more of a fair comparison than FFXI) both priest and monk, unless something has changed, can heal while doing damage. Translated to FFXIV this would be the equivalent of regen/fairy. The heals would be enough to sustain through auto-attacks, and through the last of trash mobs, while the casting part would be for more serious healing. Give RDM/DNC it's own cleric's stance where it transfer's damage done as healing and we've got a very desirable healer.
I just really cannot find any reason to bring RDM into the tanking slot when there exist a mage who iconicly get's stronger by getting hit. If they were to have completely sworn off BLU with a 100 percent confirm I'd change stance a bit, but before it was just a "not likely" and now it's even mentioned in the rising event.
Also, SE is known to take the easier route with this game. I believe in a live letter they even stated they wanted to keep tanks' in the same gear type, and I'm pretty sure a RDM in heavy armor instead of pimp hat and coat would be distasteful, whereas BLU and SAM would look sweet in heavy armors and furs/hides.
Just my two cents. Gave it two days of honest thought, but ultimately I'd want RDM to spice up healing before mucking up tanking.