Even Decade Complete seems more like a "tribute" form than Complete GokaiOh is.
That's because Kanzen Gokaioh is supposed to be the Gokaiger's own powers, not other people's. Of course, when you look at it, it uses the arms of the Abaranger Great Power and the legs and other parts from an Engine, which, rather than following the Gokaiger around, has to be summoned with the Go-onger's Great Power every time... So, that idea wasn't executed very well.
Really, the only way they'd be able to pull a thematically fitting all mecha combination would be if the Gokaiger had gotten a second main robot created by combining all the Great Powers, rather than just 3 like the Gozyujin and then combined that with the Gokaioh for the 35 Great Powers combined into one. That would also help explaining the comments about the powers needing to be together right now, although they'll hopefully explain that later.
Alternatively, it'd also work if the Gozyujin had been a completely original mecha, like the Gokaioh, rather than being something supposed to represent 3 Great Powers.