Future expansions.

I highly doubt they'll add Phoenixes.

I agree... But it doesn't hurt to hope. :)

Why would they suddenly add them to a faction?

Well, Phoenixes have always been on the same side as the forest army in "Heroes." Plus, they fit well with them. The question is, according to the rules of Ashan, Phoenixes should remain neutral. But I see them in Sylvan. As well as water spirits. It's just not meant to be.