There were a lot of complaints about Tsukasa's depths of hidden knowledge, including being the first (and so far only) human to understand the Grongi language and having extensive knowledge of the various worlds despite being an amnesiac. However, The Reveal of his identity suddenly made it all make sense: as the leader of Dai-Shocker, he would have made contact with the worlds before in order to recruit races like the Grongi, and in order to study up on the biggest threats to his ambitions - the Kamen Riders.
Although the In-UniverseHand Wave is that it's one of Tsukasa's special skills that he acquired whenever he goes to a new world, like how he could play the violin, or how he could enter the mirror world.
An equally spoilery example is the case of Otoya's unknown fate at the end of the Negative World, although...given the fact that he transformed into Dark Kiva a total of three times in both here and in Kamen Rider Kiva, we can see that he is as good as dead.
But in that world he's most likely a kaijin, so he's probably still around.
There were also complaints about the quick defeat of Great Shocker during the movie. Maybe it's because Shocker just doesn't stand a chance against an army of Kamen Riders? In fact, Tsukasa, when he's still the Great Leader, seems to even try to fix this during the beginning, hence the Rider tournament.
Diend being the focal character of the third movie of the Cho Den-O trilogy, where he is targetted by a Time Police that wants to arrest him, robbing him of his freedom. Well, remember Diend world, and what's Diend's "treasure" there again? note Freedom
Why did the Final Form Ride for Den-O change from the DenLiner to Momotaros? Because Decade didn't completely solve the problem of Den-O world.note :a.k.a. the 4th Den-O movie
The series as a whole is, as already mentioned, a Milestone Celebration of 10 years of the Heisei era. Why would it go on to include the Showa era, making it add up into a 38-year celebration note okay, okay, so maybe that is something for a Long Runner to brag about, but aren't celebrations reserved for multiples of 5s and/or 10s?? Seemingly awkward, until the Net Spinoff Videos pointed out that Decade is officially recognized as the 25th Rider hero...
Some of Diend's multi-rider summons bring forth some brilliance.
Kabuki and Rey were both good guys with dark intentions that eventually made them side with the villains. They might be to foreshadow Kaito's stance in the show.
Drake and Delta were The Gunslingers and both names start with D. They were summoned due to them being better than G3-X's power.
Sasword and Saga have Cool Swords and both names start with S. Both of which have poisonous animal motifs. They were summoned to fight Den-O's Sword form.
Ixa and Psyga are both colored white, work for an organization much like Kabuto, and at one point or another, they were bounty hunters, which is what Diend is using them for when he summons them.
Scissors and Raia are based off sea life, ironic since they are fighting a sushi maker. Also, The two riders are from a show that the same writer from Shinkenger wrote.
G3, Caucasus, and Ohja share the same colors as the Taros sans Momotaros.
Punch Hopper, Raia and Gai were killed by Kick Hopper and Ohja respectively.
Having trouble determining where All Riders vs. Dai-Shocker fits in? Look near the end of #27, in which Eijiro puts up a photo. Recognize it? It's the backdrop for Decade's World, and thus, All Riders takes place between Black RX's World and Amazon's World.
Onodera's being weaker than Godai, while frustrating, makes a lot of sense. First off, due to Tsukasa's help (or interference if you prefer) Yuusuke was force to deal with the Big Bad of his world much earlier than his original counterpart did. Well before he had the opportunity to take the numerous levels in Bad Ass that Godai needed to solo the entire Grongi race. Secondly, with the Grongi defeated, and Tsukasa being pretty much alright on his own, Yuusuke probably had less motivation to fight unless it was under certain circumstances (being driven to do so, or being asked). As result he never was able to achieve the same level of strength the original Kuuga did. He could have however, had things continued in the manner they were before Decade showed up.
Continuing on that note, Rising Ulitmate Form could have been achieved by Godai had a situation come up where he needed even more power than what Ultimate form provided (and if he hadn't broken the Arcle in the finale). After all, Kuuga is all about constantly evolving to meet the situation isn't it? Or maybe he always could upgrade Ultimate form to Rising ultimate form, but simply never got the chance (all the other forms apparently already existed) due to the events of the final battle.
Many fans were disappointed that no Evil Counterpart for Decade ever came up, especially in Movie Wars. However, there was an Evil Counterpart in the form of the Dummy Dopant. Both start with D, and assume the forms of different people. The Dummy Dopant, along with just different people also had become a Kamen Rider and a Kaijin, thus two birds (an rider who uses evil Kamen Rides, and a rider who transforms with Kaijin Rides) with one stone.
Take a look at the episode numbers. Asides from the first episode, all the episodes are two parters, with the first part on an even number, and the second part on an odd number. Then look at the fates of the worlds in context. Nearly all of them were fine and dandy before Decade came and caused some problems, usually wrecking more things than fixing them. There are some exceptions, but for the most part, Decade comes to a world that was balanced (even) and leaves the world having changed a lot of things (odd).
The Final Form Rides may make sense only in a toyetic context at first. However, considering that the Decadriver was developed by Shocker, it may have been a way to turn a Rider into a weapon for Decade to use to defeat other Riders.
Continuing this train of thought; the Final Form Ride system's main weakness is that you need the Rider in front of you to do it, kind of difficult when they're spread over the worlds. Ideally, you'd need a support system that can bring you the Riders you need...and what does the Diendriver do?
This was already addressed in Shinkenger world, where Chinomanako summons Blade to fight Tsukasa and Takeru, only for Tsukasa to turn him into Blade Blade to finish Chinomanako off.
Gaoh being absent actually has a very, very meta explanation. For one thing, the Gaoh suit was destroyed to make Yuuki in Farewell, Den-O!, so it was impossible to bring it back. To cover this, a Gaoh Ganbaride card was released in tandem with the movie. The card is faded, like Decade's cards before being active, but is also smokey and covered in ghostly chains. The general implication of this is that, because of the events of I'm Born!, Gaoh has been permanently locked out the timestream.
That doesn't explain why they just didn't use Yuuki instead, however.
Because they already had a movie-exclusive Den-O series Rider; they just happened to be fighting against him!
The Queen of the Fangires has a true name, which is "Perishing at the Altar, Silence from a Deep Kiss", which may be gibberish, but upon careful thinking, is pretty clever: Both of the Queens that the actress portrayed died just outside of an altar.
Tsukasa's catchphrase makes sense when you realize what he was supposed to do. "I'm just a passing-through Kamen Rider. Remember that." His mission was to make people remember Kamen Rider and thus save their worlds from fading into obscurity/being destroyed.
There's another reason too. When he died, he was saved because people remembered him. It was a sort of Gambit to make sure he lives through his mission.
Yuusuke's role in the Rider War note as part of the nine main Riders gunning for Decade might possibly mean that the original Kuuga world might have already been destroyed even before the start of the series...note AU!Yuusuke probably being a substitute for OU!Yuusuke
Since the Decadriver and Diendriver were created for use by Dai-Shocker, many an unsavory theory was bound to crop up regarding the Riders' Final Form Rides.
Another unsavory theory comes when you realize that the original 9 Riders were banking on Tsukasa, the leader of Daishocker, to kill them.