The further adventures of single-father Shin and his rebellious teenage grasshopper-hybrid daughter would be awesome.
Scissors is numero uno. I don't care what anybody says, he is an awesome idea that was just executed too quickly. A perfect continuation of what they had developed since Kuuga- Riders and the police. As a regular character he could have been like Ichijô, but evil! That rocks. He could have pulled off some serious stuff as the ultimate two-faced villain. You could basically have him around as long as Raia or Tiger were and still do the same conclusion, and it'd probably come off more powerfully. And at least he doesn't get killed the exact same way all the time like Ryuga does.
I would liked more Verde, but in order for him to really work in the series you'd have to have developed Zolda into more of a good guy as opposed to a not-so-bad guy. I think Verde's coolest, most-underused factor is that he's what Kitaoka could become without Goro. Inoue is fond of having stuck-up rich guys in his scripts to show how wealth and power corrupt, but when he wants to soften the blow he gives his stuck-up rich guys a funny butler/sidekick. Tsurugi has Jiiya, Kitaoka has Goro, etc. Takamizawa had nobody in the special; he wouldn't on TV either. He's one of those guys who doesn't have "friends", he has "tools."
It's also important that you introduce him into a setting where there are definite good guys, because his whole deal is about being the ultimate hypocrite: he hates having these goody two-shoes Riders joining forces, but him and all the others still basically form a clique to take them down. He wants controlled chaos. A great idea for a big villain, but he'd need to be properly built-up in the series.