CD Projekt Red have been leasing Geralt out to other franchises such as Soul Calibur VI and now Monster Hunter World. The crossover was announced a while ago, and the date Geralt will walk into a wrong portal is set to be 08 February 2018.
Capcom have announced the official date of the crossover between Witcher and Monster Hunter franchises through a tweet, where they also confirmed that even though the event will not be on PC at the same time, it will come at a later date. They didn't state the exact date or even a time window when the PC players may get the crossover though.
Another interesting bit coming directly from Capcom's responses is that players will actually play as Geralt, but they didn't explain entirely how. It could potentially be a skin or a completely new character. Some players are that Geralt will indeed be a whole new entity that players will control, rather than just a full layered set.
The reply mentions Geralt's magic as one of the main reasons why his appearance will not be just a skin, as the magic will need its own animations. This can possibly extend to Geralt as his movement and combat animations in The Witcher 3 are vastly different compared to those in Monster Hunter: World.
Grant, animations are an extremely important part of immersion, but the voice acting is also on the top of the list. To that end, players will be relieved to hear Doug Cockle reprising his role as Geralt. This is another indicator that there may be something more to the character than just a different appearance.
Higher emphasis on RPG elements and immersion is probably stemming from the close collaboration between Capcom and CD Projekt Red where they they will bring the familiar mechanics of The Witcher 3 to Monster Hunter: World.
Details are still somewhat tight on the crossover but it will not be long before everything is made plain. After all, a portal will spit Geralt out in about three weeks from the moment of writing.