-Ocean-themed Rider. I think the reason it never really gets done is the logistics of it and the fact that it's hard not to make jumping into the water every episode repetitive. This is why X-Rider dropped it early on (I can pinpoint the exact second- it's when the Jin Station blows the hell up in episode 2!) I bet it could be done, maybe Double will have a Rider from Hydro City or something.
- A member of a Sentai turns out to be evil and stays evil until the end. Like, how much of a twist would that be? They're about to combine into the robot, and suddenly one of them is like "No" and reveals they've been with the villains all along/just joined up/hate one of the others so much that they sold out to become more powerful/whatever.
I don't ever seeing it happening, unless it was signposted early and the character changed in like 2 episodes or something. I mean a really traumatic, middle/late-series change with someone we thought was a good guy all along, and they're replaced by a side character who you could just tell was being primed for hero material. It's probably a bit too grim.
- A main Kamen Rider is out of his own series for an extended period of time. Like the original series, but intentional. I'd like to see Toei try this some day, although it would have to be in the right series which had everything set up for it. The title guy's out of the picture so the not-so-trustworthy second Rider has to step up to bat and basically grow up and become a hero. The opening credits even change briefly to reflect this.
- Evil anti-matter alternate universe mirror eye patch goatee Ultraman episode where the Ultraman is evil and the monsters are the good guys: I remember someone (Brad Warner I think) actually proposing this back when Gaia was on. It's a pretty radical idea I'd like to see.
- Fourth Wall-Breaking to the extreme: there's talking to the camera, and then there's realizing you're in a fictional universe. I think it would be pretty awesome to do an episode like that. Probably Ultraman, which allows for a lot of range in the kinds of stories and has a few contenders for "mother of all acid trip" episodes.
-Musical Episode: It's like the Faiz HBV, but a full 24 minutes. I would have liked one in Decade, Tsukasa and Yuusuke get a duet and even Narutaki gets to sing about how much he hates Decade!
-A direct adaptation of the Henshin Ninja Arashi manga: On TV, Arashi's a pretty standard good guy hero-type, a Kamen Rider in ancient Japan. He's cool, he's undeniably the good guy. In the comic though, he is one chilling sonnuvabitch. My favorite moment is his solution to stopping a bunch of otter people from blowing up Osaka Castle: He waits until all but one of them swim under it to plant the explosives, kills the remaining guard, seals off the cave entrance, lets them all get blown up along with the castle and everybody in it, then makes a badass one-liner about how "I have one goal only: destroy the Blood Wheel Clan". Hard. Core.