The Snow Queen
Since the sequel is coming out next 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 love 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 more powerful.
Her main motive is set the whole of Arendelle on ice permanently, unfortunately her powers were remove by sisterly love.

She should be the Protagonist

She was possessed by 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 second theory is that maybe she could be the source 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 Queen is going to make her way into the sequel.
Anna and Elsa