>>6422819It's not exactly known. The plot should follow in rough lines what we got in the final movie, Narutaki basically announces it in the final episode if you rewatch the final moments (Decade will destroy the worlds, Decade will kill every Rider, and then Kadoya Tsukasa himself will disappear!) but it'd be very expanded.
For one, the initial announced name of the movie was Decade vs All Riders, not Decade: Final Chapter. Then there was also a second Decade character, although his function in the plot isn't known.
Also, Tackle (who wasn't in the original teaser but was meant to be in the movie) would have been the actual dead Tackle, searching for Tsukasa after Stronger's death. Stronger was cut from the movie, so they changed her plotline to what we got (mentioned in an interview with the producer).
Shocker, although not in the video teaser, was mentioned in the website's text teaser, so they were always supposed to be in.