Oroshi Kirin and Alatreon were in the leaked list of monsters for MHW before it came out. Deviljho, Xeno'Jiiva and Bazelgeuse were on that list, and we know that Deviljho was coming later, so it is possible that Oroshi Kirin and Alatreon are planned for Iceborne. I would say that Alatreon might just be the final boss of Iceborne. Though, Fatalis recently got a MHW styled picture of him done, so maybe he might be in it. I would love that, since I never got to beat him in any of the other games.