Mountains to me has similar issues to islands. The best front runners are already in game.
For mountains the: snow leopard, markhor, takin, ibex, baboon, Wolverine, and Andean bear are already in game in one way or another.
For islands the: fossa, kiwi, little blue penguin, multiple lemurs, Tasmanian Devil, and even exhibit animals like the Antilles iguana are in game.
Is possible they could still make Mountains and Islands? Yeah sure, one of the main issues though is Frontier is running out of “big mammals” which they use to bulk up packs. Call me pessimistic, but I highly doubt Frontier would be willing to start filling packs with birds now. Even reptiles for that matter. Even ignoring birds, one of the issues is that the animals remaining may be more difficult than what Frontier is willing to do. I wouldn’t expect any innovating packs any time soon.