Excluding Kamen Rider Decade and his Kamen Ride Cards, Momotaros currently holds the record for having transformed into the most Kamen Riders, having transformed into Den-O, Decade, Momorider, Geiz, Zi-O, and Woznote counting Mini Den-O as part of regular Den-O.
Cash Cow Franchise: Den-O has been living on well beyond the year-long expectancy of its fellow Rider series, and doesn't look to be dying anytime soon. Lampshaded in the opening Imagin skit in Farewell, Den-O.
"Well, for all the talk of 'Last' and 'Farewell', it's always common practice to make a sequel, isn't it?"
The Cast Showoff: Takeru Satoh portrays 1 character who's gained a total of 7 different possessed personalities (some of them by force), sings in every version of Double-Action (except Gaoh Form, Coffee Form, Climax Form, and Strike Form), his own solo song, Real-Action, and, thanks to Ryutaros, has an excuse to demonstrate his breakdancing skills.
The Other Darrin: Both Ryotaro and Hana got re-cast throughout the life of the show and its films.
Yuriko Shiratori left the production mid-way due to the stresses of filming, and was replaced by 10-year-old Tamaki Matsumoto. In the story, the alterations to the timeline made by Kai and the Imagin cut off one of the many branches of time and resulted in Hana's age being reduced to that of a child, and she was passed off as Hana's little sister "Kohana" to the Muggles.
Takeru Satoh ceased reprising his role as Ryotaro after the third movie, as he had begun seriously acting in Japanese movies (seriously, he's shown up in Rurouni Kenshin, Beck, Bakuman。, Ajin, and Inuyashiki). Takuya Mizoguchi, who was the younger Ryotaro nicknamed "Kotaro" in the Ore, Tanjou!! movie, became the main Ryotaro starting with the Kamen Rider Decade episodes set in the World of Den-O, using the same reason in-series for Hana's age reduction to explain why his own age was reduced. But whenever Kamen Rider Den-O has to show up in suit, it's usually Toshihiko Seki as Momotaros rather than Mizoguchi as Ryotaro anymore; it took until Heisei Generations FOREVER, ten years after the third movie, for Satoh to return.
Prop Recycling: The movie Riders use reworked Den-O suits; taking it a step further, Gaoh's suit was used to create Yuki, which is the reason Diend Complete Form doesn't have a Den-O representativenote The producers wanted to use Gaoh, but couldn't because they didn't have the suit anymore, so they decided to just leave Den-O unrepresented; presumably, Yuuki, a spring movie rider, was excluded due to said form being themed around summer movie riders, with the only exception (Skull) likely included a), as a replacement for Den-O, and b), Eternal had yet to make his debut
A few of the Imagin were reworked from others; for example, the Owl Imagin from Ryutaros' character focus episodes was turned into the Bluebird Imagin that appears late in the series. More obvious examples include the Leo and Albinoleo Imagin (both of whom are Kai's subordinates) and their Leo Soldiers (reworked from ZECTroopers).
The handle of the Denebick Buster (Deneb's cannon form) is a repainted version of the D-Knuckle.
Hana's actress quitting has the show replacing her with a child actress, explained as timeline changes having caused her to de-age.
Despite Takeru Satoh being unable to reprise his role as Ryotaro Nogami (at least until Kamen Rider Zi-O), Den-O has made some of the most appearances post-series thanks to Momotaros's voice actor being a lot easier to acquire compared to needing a live-action actor.
Romance on the Set: Takeru Satoh (Ryotaro) and Rina Akiyama (Naomi) dated for a while in 2008.
Star-Making Role: Of all the Kamen Rider main actors, Takeru Satoh is the actor with the most abundant post-Rider career.
What Could Have Been: Promotional materials for The Onigashima Battleship showed Ryutaros-possessed Ouja wielding G3-X's GX-05 Cerberus Gatling Gun, presumably loaned to him by an Urataros-possessed G3-X. However, in the movie, R-Ouja only wields the Ryuvolver. One possible reason for this change is that in the movie, Diend ends up summoning G3 instead of G3-X, and plans to have R-Ouja use the GX-05 were scrapped due to G3 not having access to the GX-05, as U-G3 is never seen using any of G3's weaponry. Alternatively, it could be that G3 was the rider that Diend was intended to summon all along, and the GX-05 was included in the promotional material by accident.
As hard as it might be to believe, Final CountdownWAS originally intended to be the end of Den-O. However, then along came the Chou Den-O films...and the rest is history. It was the last movie Takeru Satoh was contracted to do for Den-O, hence the reason why Takuya Mizoguchi plays Ryotaro in every one of the Chou Den-O movies and the World of Den-O, as it would have been too expensive to rehire Satoh by that point.
If Naomi reminds you of someone from another Heisei Kamen Rider Series, it's because her actress, Rina Akiyama, previously played Mana Kazaya from the first Kamen Rider series Yasuko Kobayashi wrote an episode for, Kamen Rider Agito. Coincidentally enough, the next role she would play would be in a work by Agito's writer, Toshiki Inouenote in case you're curious, said work was a cameo as a police woman in the summer movie for Kamen Rider Kiva.