It was previously announced that they'd reveal a surprise guest appearance of a Legendary Rider actor at the release of the movie. His return to the franchise is a surprise to fans since after Kamen Rider Den-O ended, the actor has gone to have a very illustrious career in Japan as he starred in multiple dramas and movies, making it impossible for him to reprise the role.
Part of the Heisei Kamen Rider 20 Work Memorial project, Kamen Rider Heisei Generations Forever film is set for a December 22nd release.
[Source: MyNavi via Tokusatsu Network]