Given that the show is targeted to the people who watched the movies or at most read the book (not to mention those who did neither) I'd go for Gandalf. He was the fire wizard and I don't remember if it was pointed in the movies that his power over fire came from Narya. And of course he mentioned the Secret Flame like you pointed.

The more crazy theory I've heard is that the Stranger is a Balrog (or maybe even the Balrog) - repentant or maybe not.