It's a little odd for BLM's job flavor that they're able to cast only ice/fire spells the entire time instead of switching between them. Instead of trying to balance the "stances", I think it would be more interesting and flavorful if BLM in PvP was brought more in line with the PvE kit (or the old pre-6.1 kit) where BLM has an ice phase and a fire phase. Here's how it could look.

An additional effect added to Superflare:

Enemies under the effect of Umbral Freeze are afflicted with Ice Immunity.
Ice Immunity: prevents accumulation of Umbral Freeze stacks for 10 seconds.

Enemies under the effect of Astral Warmth are afflicted with Fire Immunity.
Fire Immunity: prevents accumulation of Astral Warmth stacks for 10 seconds.

Also, the damage of the main GCD spells should be adjusted so fire emphasizes damage and ice emphasizes control.

Fire: 6000
Fire IV: 8000
Flare: 12000 (8000)

Blizzard: 4000
Blizzard IV: 6000
Freeze: 8000 (6000)

Thoughts? It could raise the skill cap on BLM by adding a dimension of having to manage immunities and timing when you want to be using Ice spells vs. Fire spells. For example, you may want to use Fire when harassing the enemy, but Ice in a teamfight where your teammates can take advantage of the stuns.