I would assume we'll see it in Paladin v Siren as it's probably water related.
My assumption would be that we won't see it in PvS, or that it will be little more than a teaser. As far as I know, Sirens don't have any connection to ice specifically (only water), and based on the upcoming Siren's appearance, she seems to be focused more on underwater creatures than ice ones.
Then there's the talk about terrain types. I suppose that could be referring to ice fields or icebergs, but it would make more sense if it was simply water (as fish can't function outside of water, while ice creatures presumably could).
The Siren could dabble in both underwater and ice creatures, but based on how much hype frost has gained, I think Arcane Wonders will be releasing a pure cryomage. Hopefully that's the next major expansion.