Did you really have to remind me of that shitty scene? ;_;

Wataru was also there in the first and last episodes. I think Black was also the original actor at some point and Shoichi appeared in All Riders. Of course we also had the Chou Den-O thing were Ryotaro also appeared (I think it was child Koutaro though).
As for Movie Taisen 2010, all riders were supposed to be the real ones in that one. (Minus AR-Kuuga, but I think they had 2 Kuugas in the movie)

I can't speak for Wataru, but you could argue Kenzaki, in his travels, became more bitter. It happened in the Kamen Rider Blade novel.

Doesn't explain Shinji, the technical pacifist, all of a sudden working with other Riders to kill Decade.

I hate that stupid Blade novel. Making Kenzaki into a bitter loner is just downright stupid.

Shinji and Shoichi are both characters that would never ever go and just kill some dude who really hasnt done anything wrong.

Exactly. It pissed me off so much.

It's what happens when you don't know what you're doing with the past characters. The Ryuki in the Rider War was most likely the original.
But it's that it doesn't really matter. Decade begins and ends on the Rider War that yields no purpose. A giant reverse button is always hit, so everything that happens in Decade or the movies that try to explain Decade don't really matter.
The Riders are out of character because Decade needs them to be so he looks more sympathetic in comparison (Or, for the time when he killed Blade and Ryuki, to look "Cool").

I also didn't get DiEnd world that much either, I can't find his motive to be all that spectcular.

Yeah... nah. Kenzaki becoming bitter is... nothing that really sounds like Kenzaki. Because honestly, if he was the type to become bitter, he would have been like that from the beginning of the show. Losing his family at young age, being betrayed by his senpai... also later in Blade so much sad stuff happened... no. Nothing could explain Decade-Kenzaki.
And I don't consider the novel canon anyway. Especially not with the drama CD also suggesting that Kenzaki is not the bitter type of character.

Me too. Like pretty much everything in Decade did.

He had his original actor back but wasnt supposed to be the orginal character. He was some AR Version of Black who never became RX and a AR Version of RX who never was Black before that and fought the Crisis Empire for decades.

Kinda mixed that up. I thought he was also in All Riders with the "real" Shoichi.

He was in all Riders but they still used Black and RX as seperate characters in that one.