Owner is old Guy Shishioh after the events of the GaoGaiGar FINAL and Project Z, after defeating all possible threats to the universe in the present but losing Mikoto in the process.Guy is contacted by The POWER and offered a new role — Guardian of Time. Being the hero he is, he accepts that role and receives the train of time, the DenLiner. By the time of Kamen Rider Den-O, Guy has already spent countless years or ages as the owner of the DenLiner and became bold and a bit eccentric. He usually hides his evoluder powers, but from time to time he manifests them in various displays of AWESOME — like when he does his old morning jog alongside the DenLiner going at cruise speed, or when he instantly knocks out the angry mob Ryuotaros caused, or when he powers the train with nothing but a custom bicycle. Alternatively, the GGG timeline is destroyed by Imagins. Nothing else needs to change.
Airi and Sakurai are distant cousinsIn the third movie, in the past the protagonists see two of Ryotaro's ancestors meet each other. One of them looks like Airi, the other one looks like Sakurai-san.
Alternatively, the protagonists were mistaken about the relationship between those two.Airi and Sakurai's ancestors never became more then Just Friends. The reason they were being targeted by the villain wasn't because they were Ryotaro's ancestors (Sakurai wasn't), but because they were Hana's ancestors.
The Imagin haven't actually been eradicated altogether.This is why they keep showing up after the finale. Kai was just a ruse to distract the Den-O crew. If anything, he was working for an even Bigger Bad. And it's very likely this Bigger Bad is a Cow Kaijin.
Den-O's two Super Modes are emblematic of the series' direction.Climax Form represents the show as a goofy comedy focusing on the Taros; Liner Form represents Ryotaro's Character Development and growth from Chew Toy to real hero. Since the Taros' antics seem to be a license to print money, and Takeru Satoh has left Den-O for other acting projects, we can expect Climax Form to show up more and more, while Liner Form's swan song was Final Countdown.
The events of Climax Deka are masterminded by Dai-Shocker.Given that Fangires aren't supposed to exist in Den-O, it was assumed that some of the Fangires were Dai-Shocker agents. This also explains how Wataru, Shizuka, and even Otoya got there.
The fact that Ryotaro is a Singularity Point means it's physically impossible for an AR Den-O to exist.Basically, the same ability that makes Ryutaro more or less immune to temporal paradoxes also prevents alternate Ryutaros from cropping up in other universes. This is why Decade went to the actual Den-O universe, rather than the World of Den-O; there is only one Den-O in the entire multiverse even if he doesn't always look like the same person.
The Denliner's different appearences in the show and in Decade are desktop themesJust like the TARDIS.
The Oni Brothers are (or were) members of Great Shocker.How else would they have mooks from a different series?
Deneb and Teddy will be added to Climax Form in the next movie.Scans have been released of Hyper Climax Form, with Deneb's head on one of the knees. Following this example, Teddy will eventually be added to the other knee, completing the look, and turning Climax Form into Kamen Rider's first ever cluster-fuck Super Mode.
Why do Imagin keep appearing even after Kai was defeated?Shiro/Yuki did it.
Sakurai Yuto is the protagonist of an earlier Kamen Rider series that was erased from the timeline.Once, there was Kamen Rider Zeronos, or '0-nos', a pun on Chronos. It was dark and gritty the way Den-O was light-hearted and campy. It involved the battles preserving Hana's future, and its climax was in its destruction. Just as (younger) Sakurai Yuto got his Zeronos transformation belt and cards from the (older) Sakurai Yuto, so did that Sakurai Yuto mysteriously get his Kamen Rider form from further down the timeline. The entirety of Den-O is actually part of the concluding movie of that series- that's why Momotaros is always at climax from beginning to the end, because that's the spot he occupies in the plot. It's a father's tale and the hardest part is knowing when to let your daughter's fate go separate from your own. Jaded and tired, the arc words "the past should give us hope" means trusting the task to even earlier and more idealistic versions of themselves. The disappearance of (older) Sakurai and the establishment of his younger self as a separate timeline is the typical bittersweet oldschool ending.
All Singularity Points can create and control Imagin.That's where they actually come from, and why the Imagin associated with Den-O didn't disappear in the end. They are the footsoldiers of a Time War, and somewhere 'down the line', there are rules of noninterference where Singularity Points are involved. Ryutaro is just unaware or in denial that he could ever be as bad as Kai.
Den-O Liner Form can't be utilized while Ryoutarou is stuck in his younger state.It certainly makes sense. Theoretically, Ryoutarou should be able to transform into Liner Form regardless of whether he's in contact with the Taros or not; he just can't use the Den-Kamen Sword's abilities without them. Despite this, he hasn't used Liner Form once since Final Countdown, even when it should be necessary (such as when he is unable to contact the Taros during the events of Episode Red). Incidentally, Final Countdown is not only the last official appearance of the Liner Form, but it was also Satou Takeru's last stint as Ryoutarou, after which Mizoguchi Takuya takes over as Ryoutarou's younger form. In other words, the only one who can transform into Liner Form is the older Ryoutarou.
Both the older and younger Ryoutarous will appear at the same time in a future installment and fight against Den-O's ultimate threatThe younger Ryoutarou will transform into Den-O Cho-Climax Form while the older one will transform into Den-O Liner Form and fight together at the same time.
Takeru Satoh will reprise his role as Ryotaro in the Let's Go All Riders movieThat's why we haven't seen the younger Ryotaro in the previews. They want to keep the surprise and also this movie looks like its bringing back the old actors so this just has to happen.
Den-Liner, Zero-Liner and all the other time trains were built by Doc BrownSee the end of Part III.
Den-O will actually stop getting/starring in movies one of these years.