New Caster Role Actions?

[Archwizard]

And then Verraise and Vercure off RDM and Devotion and Summon Titan Egi off SMN? No Role but tanks have maximum eHP-increasing actions. Leave that level of capacity, and BLM, alone, unless you're willing to take whatever balancing penalties that unique survivability advantage would cost you and would be as willing to see your own job lose ownership over its tools.

I think it's more a matter of just wanting casters in general to have an "Oh Shit" button for when the boss is about to explode the arena and kill all of the squishies. I would assume in this hypothetical that BLM would be compensated.

Granting, with Titan-Egi getting reworked from tanking to providing survivability to the caster, the only job that really has that issue is...

Would be nice if RDM had some survival utility they didn't have to sacrifice a gcd to use. But *shrug*

... Yeah, that. In which case, just give RDM a tool of its own ("En Garde", anyone?), leave Manaward where it is and we don't have to worry about SMN becoming a caster tank.

Replace Drain with Necrogenesis (Bloodbath, but for magic damage; shorter and stronger).

To be honest, I'd rather just give BLM a buffed version of Drain. If Manaward already provides BLM with snap survivability then it shouldn't use another cooldown to heal back up.
I'm totally fine with BLM sacrificing DPS in order to keep itself alive -- that's kinda the point of it being the archetypal squishy mage.

[Shurrikhan]

I think it's more a matter of just wanting casters in general to have an "Oh Shit" button for when the boss is about to explode the arena and kill all of the squishies.

But literally no role but Tanks has this at present. Neither Second Wind nor Bloodbath increase maximum eHP. They cannot protect against one-shots. We'd just end up with "kill all the squishies... except casters". That's why I mentioned Necrogenesis. That has balanced precedent. An HP shield does not.

On the offhand chance you'd actually use Titan Egi just to survive something as SMN, you'd still be losing not only your RDM to burst raid damage, but also your Bard, Machinist, Dancer, DRG, your NIN if the damage is magical, and likely your SAM and MNK as well. RDM would not be the only DPS without significant max eHP increase; that is literally unique to BLM at present. So don't try to pass that off as a Caster-vs-Everyone-Else disparity. It is a tool unique to ONE job. Only two (soon three, though clunky and costly) other DPS even have any personal anti-magic mitigation whatsoever.