Kamen Rider x Super Sentai: The Great Superhero War/WMG

This movie will be epic
Well duh, but until it happens this is a guess.
 * Pretty much jossed given the apparent trainwreck of a plot.
 * Which isn't actuall jossing. Something that sounds crap on paper can still be epic.
 * YMMV. Early reviews point out the plot as being especially problematic. Whether its epic or not depends on whether you care at all about plot or not.
 * The plot in these things is always secondary to the mere spectacle of the team up, in this case times a couple of hundred.

Kikaider, Kikaider 01, Inazuman, and Kaiketsu Zubat will appear in this movie
They appeared in the Kamen Rider 40th anniversary movie.
 * Jossed.

Metal Heroes will appear in this movie
Gavan appeared in a Gokaiger movie.
 * Jossed.

Takeru will appear in this movie.
He's had history with Decade before, so it wouldn't come across as much of a surprise to see him, really.
 * Jossed.
 * Proof of this being Jossed?
 * Of course Takeru appears, he's a red ranger. Unfortunately, he only appears transformed as Shinken Red, and with a The Other Darrin voice.
 * OP here -- I meant unmorphed, so yeah. I'm sad now.

Why 240 heroes?
Excluding the extra Riders, there are only 27 Kamen Riders. If you do include the extra Riders, there are a lot more than 40 of them. So this spot on the WMG page is for the theories you can come up with on how Toei is even going to actually pull off 240 heroes.
 * While there are definitely a lot more Kamen Riders than just 40, it's possible that those that were just in a movie/1-2 episodes/villain role were cut. 1-hit wonders and evil Rangers didn't make it into Super Sentai's 199, so neither will those equivalents of Kamen Rider make into their 40. I believe they're only going to count main Riders (currently 28, including Fourze), and main secondary Riders only (such as G3, Gatack, Diend, Meteor, Birth, etc.). I believe there's only 12 Secondary Riders, given that they became a tradition after Agito.
 * Eh... there may be exceptions to that rule, like Odin possibly showing up alongside Ryuki and Knight, or Lengale and Challice, as well as Blade and Garren, showing up to help.
 * Apparently there will only be 171 Sentai warriors in the movie (176 if we count the Dekaranger SWAT modes separately). With the exception of Big One, Gokai Silver and the green and black duos from Liveman and Go-onger, none of the extra rangers will be featured (not even Three Plus Two additions from Hurricaneger, Abaranger and Gekiranger).

The Gokaigers and Go-Busters will have no interaction with each other in this movie.
Cause they'll want to save that for the vs. special in January.
 * Jossed already: there's an image of the Gokaigers stepping in to help the Go-Busters against Decade.

After this movie there will be another Super Hero Taisen, with Metal Heroes joining Sentai and Kamen Rider.
There are roughly 50 Metal Heroes, add that to the 240, and take in a few of whatever new heroes are coming up in Sentai and Kamen Rider, and we could get 300.

Both Captain Marvelous and Tsukasa Kadoya will be Brainwashed and Crazy

 * They would probably be under the control of both Shine and The Great Leader.
 * Or maybe under Doctor G. There is a scan shows a monster-version of him. Why else did he have such scan?

Marvelous will use Basco's trumpet to try and take the greater power of the Kamen Riders.

 * It's known that he is out looking for it, and it's doubtful that Marvelous in his current state will do the usual routine of them eventually giving it to him by choice.

There will be a Shout-Out TO Mighty Morphin Power Rangers and Masked Rider.

 * in the form of having Kyoryu Sentai Zyuranger fighting Black, RX. J, and Zo with a slightly Take That on how Masked Rider used the suits.
 * Jossed, but Super Hero Taihen did make a Power Rangers reference.

Marvelous will transform into Red Turbo and Mega Red during his battle with Fourze.
Just as how Tsukasa will transform into Riders who are similar to the Go-Busters while fighting them (Faiz and Kabuto's powers originate from a company like the Go-Busters, as well as Kabuto and Red Buster's Super Speed abilities), it would only make sense that Marvelous transform into Red Rangers who are similar to Fourze. Think about it: both the Turborangers and Megarangers were High School students during their respective tenures like Fourze is now. Next, all three heroes were not the brightest bulbs of their schools. Finally, Mega Red and Fourze both have a drill weapon (Kenta's Drill Saber and Gentaro's Drill Switch).
 * Jossed. The previews reveal that Marvelous will use against  respectively.

The ones behind all this are Narutaki and Basco.
Narutaki still wants Decade out of the way, and Basco (brought back to life by the magic of crossovers) still wants a Great Treasure. If Gokaiger is anything to go by, using the Kamen Rider universe's Great Treasure will erase the Kamen Riders from existence. So the two cut a deal; Basco gets the Treasure, Narutaki gets to take over the KR universe after the Riders are dealt with. But in order to get those pesky Sentai out of the way, they engineered the Let's You and Him Fight situation in the hopes that the Riders would take out the Rangers. Or, alternately, the Marvelous we've been seeing so far is Basco in disguise and the real Marvelous will return about halfway through the movie.

The Great Leader is behind it all and has taken possession of Decade and Gokai Red.
The Big Machine project was to trick the Heroes into fighting each other. While they were fighting, the two evil organizations were working on conquering the universe. They combined their battleships, Crisis Fortress and Gigant Horse, to create the Big Machine.

Big Machine is the original Kamen Rider manga is the name of Shocker's Great Leader.

Beet Buster and Stag Buster will make their Early-Bird Cameo here.
173 Sentai warriors have been confirmed for the movie. This excludes all of the sixth rangers and extra heroes outside of Gokai Silver, as well as some of the non-sixth extras such as the Gouraigers, Abare Black & Killer and Geki Violet & Chopper. This means there's one team with six members, five teams with three members (15, counting the initial Go-Busters roster) and thirty teams with five members (150). Since that only adds up to 171, that leaves two Sentai warriors unaccounted, which means that the extra members of the Go-Busters will make an appearance here, much like Gokai Silver did in 199 Heroes.
 * Disconfirmed. The two unaccounted rangers are Geki Violet and Chopper, who appear in a flashback only.

Tsukasa is using Dai-Shocker, same as last time
Seriously, given what happened in that movie, do you think he'll be so quick to trust them again? They literally kicked him out in favour of his sister and Shadow Moon.

Loki is behind everything.
You thought it was a coincidence that two big superhero crossover movies came out so close together? Loki who has demonstrated the ability to revive the dead in the comics (being the Father of Hela has it's advantages) resurrected the villains of the various Sentai and Rider series to keep Japan's vast array of heroes busy while he invaded the rest of the world. Once he had the Tesseract and established a foothold in North America (who at the time had only a handful of unorganized heroes) he would have had more of a chance against the Japanese heroes. Besides doesn't pitting two factions of heroes into killing each other sound like something Loki would do?