The Second Countdown at Kamen Rider Web was related to the reboot of Kamen Rider BLACK as "Kamen Rider BLACK SUN". This isn't going to be a movie, but a series. Probably similar to Kamen Rider Amazons and it is aimed at Adult viewers.
Filmmaker Kazuya Shiraishi who is known for his films "The Blood of Wolves", "The Devil's Path" is going to direct the series.
Comment by Kazuya Shiraishi
“The thought of being in charge of this amazing reboot of Kamen Rider Black gives me the shivers. I’m going to need to rely on every bit of my skill to ensure I don’t get crushed by 50 years of Kamen Rider history. I want the tragic story of Kohtaro Minami and Nobuhiko Akizuki to carve a new niche in the chronicles of Japanese heroes. I plan on giving it my best, I hope everyone will see it!”
Though this series is going to be part of Kamen Rider 50th Anniversary project, but it will be released in Spring of 2022.