Red Mages Needs a Bit More Complexity

[Galvuu]

Depth would be something like Verstone/fire: The next spell you cast of the opposing element deals 5% more damage. Even then that's not really much depth to add to it. Or something like, becoming unbalanced in mana reduces the damage of spells of the opposite element by 10%. There's really not a huge amount of depth that can be added to it without serious changes.

This is actually a pretty good idea in a vacuum. The only sad part is that RDM already naturally alternates between the two types of spells like this to keep the mana mostly even.
Like you said, you'd need some bigger, more structural changes to add this complexity.