Would love to see a narrative story starting in the streets, in Arkham Horror 3e and the story ending, where you need to play the next part in Mansions of Madness 2nd edition. This would be fantastic collaboration between the two games and lead to a fantastic story over 2 play nights, with 2 different games.

That sounds great except for the issue that people who only play one of the games would be alienated by that and feel like FFG was forcing them to buy an entirely separate game when being a completionist for one of their games already carries a significant buy in cost.

If you want a multi-part adventure/campaign, I highly recommend the AH card game.

Also peoples own house made scenarios could use that idea.... but yeah AH Lcg fills that role quite well.