I can see her having a home in handbuff and mulligan elves, which both build up big late game finishers they'd REALLY like scorch protection for. While I understand mogwai's math on her, I feel it's a little off. If I Morenn a Scorch that would have killed my 25+ creature, being able to aglais that scorch isn't going to help if the next biggest creature is a 9 or even worse, is still mine.
The big issue is that Gigni still exists, and is most people's choice of removal. I really like that she's a thing, and I can see her seeing common use, if not enough to make ambush a worthwhile mechanic. I think she'll be a grower, too, as more specials enter the game over time and more people play them. Now, a turn 2 opener of Morenn is going to be a pain for some decks, especially weather decks who really want to throw down a lot of stuff quick.