Monster Hunter: World has enjoyed loads of free updates since it launched on PC in August 2019. These seem tiny in comparison to the upcoming Iceborne expansion, which promises to be "massive", and comparable in size to the old Ultimate and G Rank versions. Capcom hasn't detailed much of Iceborne yet but we are expecting more information in March 2019 when Capcom celebrates the series' 15th anniversary.
The Iceborne expansion is due 'Autumn 2019' but a tiny asterisked statement on the announcement says "*Steam release follows", which suggests a possible release delay on PC. Monster Hunter: World released on PC seven months after consoles.
All we have so far is this quick teaser video that shows Rathalos flying off to an icy land.
Iceborne will continue Monster Hunter: World's story with new locales, moves, equipment, a new quest rank and monsters. New monsters weapons and an elite 'G-Rank' are standard additions for a Monster Hunter game, but the mention of new moves is interesting. Expanding movesets for existing weapons would be a big deal for players who have settled into a weapon's style over the course of hundreds of hunts.
According to the official Iceborne site more information will be revealed 'Spring 2019'. Monster Hunter's 15th anniversary is coming up in March, so we expect more information then. Capcom recently launched a 15th anniversary website which might reveal more features. For now there's nothing there except a big monster eye.
Nothing has been officially revealed yet, but the red eyes in the teaser trailer almost certainly belong to Nargacuga. In previous Monster Hunter games Nargacuga's eyes glow when it enters its rage state. It's a mid-sized predator wyvern that tends to live in wooded areas. That suggests the new expansion's zones may not all be as icy as the trailer implies.
That's as much as we can glean from the official announcements for now. We'll update this page with everything Capcom announces this spring.