Maybe, Onodera is considered the real Kuuga and Godai from the actual Kuuga tv series is considered just a secondary Rider of the original Agito World?
That's an interesting theory, but the way the Kuuga arc went down in Decade doesn't bear it out, I think. The Kuuga arc is the only one that bothers to explain WHY a different character than the canonical one obtained powers. That makes me think something more complicated (or purposeful) is going on there.
The audience is very clearly shown Onodera getting the Arkull belt because Narutaki gives it to him. Yashiro has a line of dialogue that even clearly references another core member of the original Kuuga cast, the translator Sakurako.
To me that makes it seem that the writers wanted the audience to know that the original Kuuga cast members still existed but had been moved off-camera in this world, possibly by Narutaki's manipulations. For instance, Narutaki could well have given Onodera the belt in order to keep it out of Godai's hands.
Given the events of the last episode, there is a possible motive for why Narutaki would choose to do this. We know Narutaki wants to see Decade destroyed or removed from the big picture. We also know Kivara is Narutaki's agent and that she is instrumental in causing Onodera to transform into black-eyes Ultimate Kuuga.
Any fan of the original Kuuga knows that his black-eyes form is one in which the Kuuga powers have consumed the bearer and turned him into an engine of destruction. Onodera proves himself a less capable Kuuga than Godai at many points in Decade-- so it's reasonable to think that Narutaki may have chosen him because he
wanted the Arkull belt in the hands of someone who he believed couldn't overcome the Kuuga power.
Then presumably all Narutaki has to do to destroy Decade is have a mechanism on hand that turns Onodera into black-eyes Ultimate Kuuga in a moment when he'll be pointed roughly in Decade's direction. All of this happens in the final episode, with Kivaara fulfilling her initially established role as Narutaki's agent.
If Onodera was "created" by Narutaki while the other A.R.s just kind of happened, that could also explain why something sustained him after the A. R. worlds apparently evaporated. Narutaki would have good reason to keep Onodera from disappearing just long enough for him to destroy Decade as the vessel of the Ultimate Kuuga power.
The main hole in my theory is that the show still doesn't clearly establish who the hell Narutaki is, what he really wants, or what his powers really are. I'm just supposing he ultimately wants to kill/destroy Decade and is generally capable of doing the things the plot requires. Narutaki could be revealed to be something else entirely and all the Kuuga stuff could be coincidental.
But Decade has consistently recognized that Kuuga is "special" among the Heisei series and seems to pre-suppose that its viewers know the basics of Kuuga's plot. It seems reasonable to assume there's some underlying reason for that and some reason for Onodera beyond Godai's actor being unavailable.
