This is just my personal opinion/theory.

I believe if Iceheart is Shiva, that somehow she is transformed into a Primal, that we'll finally see the process of how a Primal is made, which could open up the doors to new possibilities for us in the future for many different things. I think the heretics aren't all what they seem, Iceheart might actually have some kind of intel that Ishgard is corrupted by some unknown force (or she knows it's by the paragons) and is trying to start her own rebellion through these "heretics". Her followers get so devout over her, and the process of how to create a primal is imbued upon her, that she becomes Shiva, in hopes to thwart what ever plans Ishgard might have against the rest of the world. I'm not entirely convinced she is the same as Ifrit, Garuda, and Titan, but more like Ramuh and her fight will be something different that just "stopping the raging Primal from sucking up Aether." Again... just a thought.