The Snow reyna
Since the sequel is coming out susunod year, here are my personal opinions whether another character with snow powers will be feature in the sequel and whether or not shall will be the protagonist or the antagonist.

She should be the Antagonist

I would pag-ibig to see Anna and Elsa having a wicked relative, who is actually their aunt and even has ice powers.But unlike Elsa's, their aunt's ice powers is madami powerful.
Her main motive is set the whole of Arendelle on ice permanently, unfortunately her powers were remove sa pamamagitan ng sisterly love.

She should be the Protagonist

She was possessed sa pamamagitan ng an evil spirit and as mentioned, Anna and Elsa help their aunt to eliminate the evil spirit that gave their aunt the ice powers, thus reconciliation between aunt and her 2 nieces through love.
My segundo theory is that maybe she could be the pinagmulan for Elsa's powers, and since Elsa could control it, she is going to control her aunt's ice powers.

The Cold Never Bother Me Anyway!

So, here are my thoughts on whether the actual Ice reyna is going to make her way into the sequel.
Anna and Elsa