From this point on, it'd be interesting to see if Toei will up the ante with the next inevitable Den-O movie/series of movies. How about Episode Green featuring V3? Episode Black featuring BLACK? Episode Bluey-Purplish featurning Hibiki? The sky's the limit. Of course Diend, like Decade, is a unique case, given how steeped in Rider mythology the character is. It's practically his whole reason to exist.
Diend is a strange character; in many ways his inclusion made Decade's narrative infinitely more complex, as it added in another mystery man who knew about the hero's back story but refused to tell, and who had his own world-hopping abilities and motivation. We did find out a little about him along the way, but he's still largely an enigma by the end of the series and subsequent movies. What's more, he has some of the oddest characterization in Kamen Rider, appearing to regress with each appearance back to the core concept of "world-hopping treasure hunter" despite whatever he'd just been through in the previous episodes.
