Acting for Two - Shoma Yamamoto played Takato Shiramine/Kamen Rider Rey in the Kiva movie, yet he appears in the television series as Taiga Nobori/Kamen Rider Saga, who has no connection to Takato/Rey.
Though given that "rey" is Spanish for "king" it makes for a bit of subtle foreshadowing.
But in this case, it was derived from a Japanese word for "cold"
And Yuria Haga in the role of Mio takes a fatal Rider Kick from one Rider to defend another one, just like she did in Kamen Rider Faiz. Interestingly, the roles of the Riders are inverted in each season.
Crosses into Take That! with Rook, as the IXA system's henshin requires depressing the IXA Knuckle just like the Sazer Knuckles of Gransazer. Not only was Rook's actor previously Sazer Tawlon, but he gets finished off by IXA eventually. It's worth noting here that Yuri's henshin pose seems to be modelled on that of the Gransazers' Flame Tribe.
Kamen Rider Ryuki had an escaped convict Rider named Takeshi. Kiva had another escaped convict Rider in The Movie named Takashi. Close enough.
Does Shima's BlueSky group make CGI animated kids' movies for revenue?
Talking to Himself: Kiva has both played this straight and subverted it: In the main series, Kivat-Bat III and his father, Kivat-Bat II, are both voiced by Tomokazu Sugita, and the two Kivats were reunited near the end of the series. However, while Norio Wakamoto voiced both Arc Kivat and Rey Kivat in The Movie, the two never interacted with each other.
Real Life Writes the Plot: The reason Kiva's four base forms were essentially abandoned in the latter half of the series was because the suits were too heavy for suit actor Seiji Takaiwa. Thus, Kiva had to rely on the lighter Emperor Form suit for most of the series.