Red Mage as possible new tank class

[TouchandFeel]

But I am not sure what other role you fill as someone who performs white and black magic either.

Exactly, that is one of the reasons that I feel that in their more "classic" form Red Mages wouldn't really work so well in FFXIV since they wouldn't fit so great in any specific role.
That is why I suggested mixing Red Mage with Rune Fencer, which was a magic based tank from FFXI that used magical wards and spells to provide defense.

One other possible way for a magic based tank to be able to soak damage is to have an ability that redirects damage from taking away HP to taking away MP for a period of time.