Shared by the publisher on Twitter, the teaser image shows a spider-web forming a giant four, symbols associated with both Spider-Man and the Fantastic Four. Spider-Man has interacted with the team a lot over the years, and he's close friends with Johnny Storm/Human Torch When Johnny temporarily died preventing an invasion from the Negative Zone, Spider-Man even took his place on the team. Over the years there have been various series focusing on Spider-Man teaming up with the Fantastic Four.
Retweeting the original post, Marvel Editor-in-Chief C.B. Cebulski said, "No one is going to see this one coming."
There's some speculation the announcement might be related to a previously posted picture teasing Ghost Rider, as the two were part of the New Fantastic Four. Exactly which Ghost Rider the image is teasing is unclear. Created by Walt Simonson and Arthur Adams, that team existed briefly in the early '90s and also featured Wolverine, Hulk and Sleepwalker.
No further information about the project -- including the title, release date or creative teams -- was included with the teaser. However, Dan Slott is currently writing Fantastic Four, while Nick Spencer is writing Amazing Spider-Man, so this may be related to one of their runs. However, it's also possible that this is a new ongoing or limited series.