In a lot of ways, this is the Den-O movie that finally really delivers on a facet of the series that up until now has mostly gone unexplored: presumably, they can meet anybody in history, or at least Rider history. It's a trait shared with Decade, though while that show is built around the concept, Den-O's only briefly toyed with it in the past.
In the previous team-ups with Kiva and Decade, the guest Riders were basically doing cameo appearances. Here however, Diend is definitely integral to the plot and entrenched into the Den-O mythos. The movie simply wouldn't be the same without him. It's still a Den-O movie featuring Diend, but he's the first name in the credits and it's not hard to see why.
After all, the main selling point of the Trilogy, beyond the idea of 3 Rider movies spread out across 6 weeks, is that each one focuses on a Rider other than Den-O. Although so far, they've still been Riders to come out of the Den-O series/movies, whereas finally we have a character from a different series, albeit one that has crossed over with Den-O in the past and has the potential to cross over with it easily.
