You know, Decade suffers from Hibiki Syndrome - change of writers midway (though reportedly one planned all along) doesn't help with the consistence of the show.
What I've read is that Aikawa was never contracted to provide any material for Decade beyond the first 13 episodes. In light of this, I feel that it's entirely possible that he didn't actually provide Toei with a full outline of the show. Toei may not have paid him to do it!
Decade goes through a long period of instability with its writing staff at the show's midpoint. It's not until episode 26 (the Black arc) that Yonemura takes over to bring the series to its conclusion. This is, not coincidentally, the point at which the themes of the Decade movies start to enter the show.
In the interim, episodes are written by Yasuko Kobayashi (Shinkenger, Den-O), Kenji Konuta (Kabuto), and Toshiki Inoue (Diend, Nega World). Yonemura steps in for the Hibiki arc only during this period.
This is not a state of affairs that suggests, to me, that Toei had a solid plan in mind for what to do with Decade after Aikawa left. Even if Aikawa had left detailed notes on how to end the show, whoever took over as head writer wouldn't have been obligated to honor them, anyway.
I will note here, though, that a critical weakness in my analysis is that I'm not taking director and producer information into account. I don't have an easily readable source for that on hand at the moment, unfortunately.
Blade - This was sorta hard.
Blade was a series tormented by production problems, up to and including the director having a nervous breakdown toward the end of the show (or so I've yeard).
In light of this, I got the feeling that Decade was having Tsukasa engage with something interesting Blade
could've been about, given the original material's inconsistency.
I think this was a good decision in the long run. Moreso than some other arcs, it was very easy to see with the Blade arc how it related to Tsukasa's treatment of his friends and how he viewed his "job" as a Rider.
Den-o - This was also hard.
The Den-O arc, I feel, fundamentally doesn't behave like the other Decade arcs for one major reason. It is, to my knowledge, the only arc written by the head writer of the original show in question.
So while all of the other continuities have clearly been reconcepted a bit, the Den-O of Decade is clearly just the same ol' Den-O that Kobayashi wrote about a few years before.
While this makes the behavior of the Imajin consistent, it also kind of renders Tsukasa and the others as passengers in a story that's really meant to promote the then-upcoming Den-O flick.
There is nominally a thing Tsukasa learns to bond with Momotaros or whatever, but the whole exercise feels much less focused on illustrating his nature as a character than most other Decade arcs.