Kamen Rider Ryuki/WMG

Kagawa was Right
But not in the way he thought.

He was correct that Yui was the key to the Mirror world, but he failed to realize that her importance was from an emotional stand-point, not as an Inverse Barrier Maiden.

We've seen that in both Episode Final and the TV series that, even when she dies, the mirror world does not close. More to the point, we know that Kanzaki can REWIND TIME; if she dies too early, then he could just try again. It's implied that that's exactly what happened a few times.

The real key to preventing the deaths was simply being honest with her. Once she found out what exactly was going on, she rejected her brother's actions. She's the only person he cares about and the only person he'd likely listen to (although it did take a while), so Shirou eventually caved and stopped the Rider War.

If Kagawa had gone in this direction instead of doing what he thought he had to do, then he might have succeeded and would have had Shinji's support.

Odin is using his Survive Form whenever he appears
Odin is a Game Breaker so why not have him go into his strongest form?
 * Recently debunked by the revelation of images and details floating round the net of Odin's Survive Mode (assuming you can read/translate Japanese). Odin was just basically the most powerful Rider by default because Kanzaki wanted to be sure Yui survived at any cost. Further implied by the fact that if this were the case, his Dragon Knight counterpart Wrath would have ben able to win the war in the first two episodes instead of being held in reserve till near the end of the series, and would not have died in the singular use of his Final Vent against Kit. (Also of note is that Kase flat out says that all the Riders have a Survive Mode card in DK. But she was only able to secure two of them. What happened to the others remains a mystery to this day.)

The monster that attacked Shinji in 13 Riders was sent by Kanzaki
Shinji keeps getting involved in the Rider war, and influencing some of them (Ren and Kitaoka mainly) so he decided to kill him off. Unfortuanately, this just led to Shinji becoming Ryuki anyway.

Kamen Rider Ryuki and Kamen Rider Dragon Knight are in the same universe

 * Dragon Knight takes place before Ryuki.
 * Ryuki takes place firmly in 2002-2003. Dragon Knight is blatantly set in 2009. How can Dragon Knight have happened before Ryuki? Furthermore, if that's the case, why aren't the Ryuki Riders aware of the DK Riders or Xaviax?


 * Ventara and The Mirror World are the same.
 * Not possible. The way the Mirror World acts is so dfferent from Ventara it isn't even funny. The major difference being that the Mirror World will eventully consume any Rider if they stay in there long enough, but will devour a human in moments. Ventara on the other hand, is established as a actual world that can connect to ours via some kind of interdimensional warp accessed through reflective surfaces.
 * Shiro was discovering the Mirror World, more specifically:
 * If that were the case, then the Ryuki Riders would have already battled Xaviax. Or at least somehow been aware of him.
 * Completely illogical. If they close off the Mirror World, all they would do is seal the portals that link the two realities. Closing a world off doesn't necessarily mean destroying it.
 * Easier explanation. The Riders systems are all Ventara counterparts to the Ryuki Riders. They were just used for completely different reasons and Xaviax is Shiro's counterpart.
 * Considering how wildly different their goals were though, the idea of Xaviax being Shiro's counterpart is implausible at best. There is, again, the fact that Ryuki and Dragon Knight are blatantly set within a specific timeframe, and have no logical connection to each other beyond the cosmetic. Ryuki was set in 2002-3, while Dragon Knight is set in 2009. Not to mention these shows taking place on wholly different sides of the planet, and for all the weird abilities Shiro displayed, he doesn't come close to doing half the things Xaviax is apparently capable of. Likewise, we don't see Xaviax, while in possession of Wrath, using the Time Vent card and we can assume from that, that while Shiro could screw with time to keep his sister alive, Xaviax either doesn't have that ability or just never used it. (one would also logically have to think that if he did have such capability, he would have just used it to restore Karsh to being able to support life, instead of going through the whole mess with Ventara and the Riders to begin with!)
 * Considering how wildly different their goals were though, the idea of Xaviax being Shiro's counterpart is implausible at best. There is, again, the fact that Ryuki and Dragon Knight are blatantly set within a specific timeframe, and have no logical connection to each other beyond the cosmetic. Ryuki was set in 2002-3, while Dragon Knight is set in 2009. Not to mention these shows taking place on wholly different sides of the planet, and for all the weird abilities Shiro displayed, he doesn't come close to doing half the things Xaviax is apparently capable of. Likewise, we don't see Xaviax, while in possession of Wrath, using the Time Vent card and we can assume from that, that while Shiro could screw with time to keep his sister alive, Xaviax either doesn't have that ability or just never used it. (one would also logically have to think that if he did have such capability, he would have just used it to restore Karsh to being able to support life, instead of going through the whole mess with Ventara and the Riders to begin with!)

The Episode Final iteration of the Rider War takes place in Agito's world.
There's probably a reason why Agito's cast makes a cameo, all of them pretty much in character.

Dragon Knight and Ryuki are in the same universe
Simply put, Ryuki happens first, while at the same time, Shirou's ventaran counterpart (Eubulon in this case) makes the ventaran decks, but they only show up at Earth in 2009, years after the Rider War, and due to being Ventaran decks, they take the user to Ventara instead of the Mirror World. Of course, because the Ventaran decks are merely copies of the Ryuki ones, they lack some things (Time Vent, for example) but have more of others (Survive, reportedly)
 * Again, not possible. Remember that aside from the fact that these shows take place seven years apart, Eubulon created his Riders sixty years before Dragon Knight begins. That would mean that Eubulon's Riders are the originals. If one wanted to place these in the same reality, one would possibly be able to make the case that Kanzaki somehow stumbled onto knowledge of the Decks, and was able to create replicas of them, though with a few more goodies of his own invention, and instead of setting up a portal to another planet across the galaxy linked to Earth by mirrors, he created a bastardized 'mirror world' where the Riders could battle. Though why he would let the mirror world devour Riders when it would be so much easier to just let them fight to their hearts content until one was left standing and then kick their ass with Odin, who could say? But in any case, in order for these two shows to exist in the same universe, Dragon Knight's Decks, and much of the backstory, have to happen first just because of the canon established in Dragon Knight.

The show ending is not the true ending.
Even if the powers that be have said that it is, and it sure looks like it should be, there is one obvious problem: every appearance of Ryuki outside Ryuki. If a Reset Button ending is the final iteration of the events, that kinda makes teamups tough. Therefore, the true ending must be a version that allows for the existence of Ryuki.
 * Well there's always Alternate Realities to consider.