Well Toei completely dropped teh ball with Decade. They could have used the series as a Crisis on Infinite Earths style story to combine all of the Heisei universes and Showa universe into one world, but nope, they just completely jacked it up because they had no idea what to do with the show.

I think Ryuki could ever fit into any series if Toei pulls a End of Evangelion on us and says the Ryuki Final is now the real ending to the series. Thus ignoring the TV show ending. I personally didn't mind the TV show ending and it didn't make me angry like most people did.

What is the Ryuki Final ending?

Shinji and Ren running off to fight an enormous army of flying monsters.

More seriously, the Ryuki Final ending is Ryuki's movie, which had Ryuga and Femme in it. Like the name suggests, it's an alternate ending.

I would probably like Ryuki Final more if Femme and Ryuga hadn't been in it. Femme was mostly annoying and Ryuga was boring.