[Librarian]

Movie rider is likely, maybe the second movie. Or a later special. Was some brief discussion about a VCR type rider that got pushed to the back burner. (Which might come back around because of how the ride watches are similar to the tape wheels in a VHS.)

But series-wise, the past connection rider is all the previous riders.

The trinity riders will be 2 future one present. For clock based, there likely won't be more than 3. In watchmakers there's something known as the "holy trinity" of watch brands. So they'll likely lean on that with only 3 main watch riders.

Though it would seem likely to do a glitchy one or otherwise not as prominent one too for a brief story(a knock off watch). Nothing that'll become a regular cameo though. More a brief one off, with maybe more appearances if audiences respond positively to it. Since other than Zi-O(and really his too since Woz gave it to him), the belts come from the future. The closest we may get is a prototype belt that's the pre-cursor to what became the Jikuu Driver. That was one of the story details in Build's appearances. His tech analysis in the first episode was pointing out that the Jikuu driver is far beyond this time period. By potentially decades.

That could be the 2038 rider. Since it's the '20 years later' rider, and Aqua is supposed to be 2051. So from the belt leaks, the 2038 rider is an unknown, and seemingly picked that year deliberately since it's directly two decades from 2018. Unless that's a future Oma Zi-O ride watch.

Anything before 2018 wouldn't be a Jikuu driver if used, and since the show is built on that, seems more plausible as a movie side rider.

There's also no "past Rider" because technically speaking as of next series, 'present day' Zi-O is the past rider. He's past and present right now since he'll transition between two eras. Which fits the king theme too. "Once and future king".