Kouhei Takeda who plays Kazumi Sawatari / Kamen Rider Grease in Kamen Rider Build is confirmed to appear in Kamen Rider Heisei Generations FOREVER. He had posted a Instagram Video where he is doing the Hair make and one of the tag being Heisei Generations ver.
It is still not known whether he'll transform into Kamen Rider Grease or not because of the effect of the New World. But since there was Grease Ride Watch released, there some chances of him transforming into it.
Eh!?... Wait! It's ZI-O, right? Could it be Kazumin from the Past who is going to appear?
Hope there release some of the filming pics in their social medias.
By thinking of it, two days back Kouhei Takeda & Eiji Akaso posted some videos where the four Build Riders are seen together for lunch.
In this Kensei Mikami (Gentoku Himuro / Kamen Rider Rogue) was also seen. If this was after the filming of Heisei Generations FOREVER then even he is confirmed to appear.