New Caster Role Actions?

[Archwizard]

More specifically...

• 10% mitigation is FAR less valuable than a 30% HP shield. One's as good as a DRK's self-cast TBN; the other is unlikely ever to save you from an attack would otherwise kill you.
• The closest equivalent to BLM's Manaward is Ninja's Shadeshift, except Shadeshift only affects Physical Damage and thus almost never affects any damage except standing in a boss's cleave, and for does so 33% less often and for 33% less HP. BLM's Manaward is uniquely powerful.

SAM and MNK's 10% mitigation skills can be kept up for large periods, which is a substantial EHP buff over the entire encounter rather than small periods (and even with giving up DPS to maintain it, it's no different from the expected scenario with Titan-Egi).
BRD, MCH and DNC, meanwhile, can extend similar 10% mitigation effects to the entire party through Troubadour, Tactician and Shield Samba, which can often be the difference between life and death for a lot of AoEs.
As Zyneste pointed out, NIN's Shade Shift no longer is limited to Physical effects in ShB, giving the same effective EHP as Manaward.

And as he also points out, Melee have higher HP as a whole than Casters, which highlights a particular weakness of RDM, the hybrid melee-caster.

I don't want it to be a RDM exclusive because I think that defensive utility like that should be shared among the role, like second wind is.

I think whatever defensive skill RDM has should play to its particular strengths and weaknesses. That is to say, should play into the fact that it regularly enters melee range of the target for brief periods, which is the period where it's most vulnerable but also playing the most as another role.