I think that they originally meant for him to be Prof. Shinigami, but since it didn't make sense and didn't have the punch of Tsukasa being the Great Leader, they tried to retcon it with Movie War 2010.
Like I said in my previous post, there's an extra scene in the Director's Cut of All Riders vs Dai-Shocker that shows his Eijiro personality wasn't just acting. He's alone in the studio and reacts with fear for Natsumi when he notices the Dai-Shocker castle rising.
Yet, a few minutes later as Shinigami, he doesn't seem to care at all when Tsukasa throws her in the sewers and just lets Dai-Shocker troops attack her without putting any effort to spare her at least. Even if they hadn't planned that he was possessed by a Gaia Memory specifically, they had planned that he was possessed by
something.
But when he first become possessed? What was the point of possessing him?
There's a flashback in Movie Wars 2010 which shows the Sonozaki's leader giving a Gaia Memory to Eijiro.
Did he know what was going on with the merging Worlds or just going with the flow? If the latter, how did he have a studio that could take the heroes through Worlds?
Well, as Shinigami, at least, he was a Dai-Shocker member. It's not very clear if Eijiro himself knew about Dai-Shocker and everything else when he wasn't being controlled. He seemed to recognize the Dai-Shocker castle though, but it's not very clear.
And why, I ask again, is everything treated with a shrug in this show? The cast literally just goes through the motions of the script without any sense of believability! I wonder if they even knew what they were doing half the time.
I've seen that lack of reaction in some anime that focus on the main character visiting weekly strange places (Kino's Journey, Mushishi). I guess they were inspired by that, although attempting to add humor too, while those shows were rather introspective and serious.
Narutaki is now a Shocker member? That seemed odd since he was trying to stop Shocker in the series.
Rewatch the scene where Tsukasa met Narutaki. "Do you hate me so much you've even become a Shocker?!". Narutaki previously believed that Tsukasa still was with Shocker, that's why he despised them (In the scene where Narutaki meets Natsumi in All Riders vs Dai-Shocker, after Tsukasawas kicked out, both still talked like they thought Tsukasa was commanding them). He had been shown working with monsters before too, like back in the Ryuuki, Blade and Hibiki Worlds.
Also, the All Riders moving taking place between episodes 29 and 30 still doesn't make sense because nothing is mentioned about it in episode 30.
Tsukasa drops his comments about not finding his own world in episodes 30 and 31 - and that's after ending 29 talking about finding his own world. That clip near the ending of episode 29 where a Dai-Shocker castle rises in the middle of a city with Narutaki watching also was directly taken from the movie (well, the castle and the city, not Narutaki). Also, it's confirmed by a Japanese publication.
It's not perfect, especially due to Decade always trying to end with a cliffhanger for the next world in the tv show, but it's there.
Plus, they should have had the riders change to their correct identities (and of course, actors). They wouldn't need to show them all, but since they had the original actors for Kiva, Blade, Hibiki, and Agito, they could have easily stated as such.
No one's identities were ever changed though. The AR Riders from Decade and the real ones are separate characters like the tv finale showed, when Kenzaki was walking around with Kendate still around.
The Decade part of Movie Wars 2010 was originally meant to be another All Riders movie, and if they had gone through that we'd probably have seen more of the og Riders like in All Riders vs Dai-Shocker, but that was changed sometime during production, so they focused on the characters from Decade itself. Also, although it's really easy to miss, the Blade that Decade beats in the first part of the movie was voiced by Kenzaki's actor.
The giant elephant is pretty much just there for a giant CG battle scene with W in the W/Decade crossover segment.
And what is up with the giant Kamen Rider? He was in the All Riders movie too.
Kamen Rider J from the movie Kamen Rider J.