I was being tongue-in-cheek about all Sentai being like that, but really dude, I think you can apply what you have there about Sentai to Kamen Rider.
I'd say the plots of the earlier shows are more different than people give them credit for. Really, out of the first 5 series, the only Riders that get turned into cyborgs against their will are the first two guys. V3, X & Amazon aren't even changed by the bad guys, and Stronger sort of volunteered himself, and is just a lucky enough bastard that he didn't get turned evil.
Sure, they're all kaizo-ningen, but back then that's just how it was all the way up to J- Kamen Riders were kaizo-ningen, period. You can blame Ishinomori for that. :biggrin: I'd argue G3 was the first "normal guy who just puts something on" Rider. Kuuga, Agito, Gills- they were all "altered humans" in some way. Same formula, but they kept changing the origin so even if they're always the moody-broody "they took away everything I had" types, they at least lost something other than their humanity- Hongo & Ichimonji were pissed because Shocker turned them into Riders against their will, but V3 wanted to become one because Destron kills his family. That's different enough for me.
Now, I'll agree the villains are pretty formulaic in the early shows- always the big guy, the commander, the monster-of-the-week, and the henchmen. But I say that's because it worked- sure it gets stale by the time of Super-1, but that's because by then you have villains who sit around eating and making monsters out of ladders and fans, not plotting evilly and making cool freakish things like Shocker. But look at the new shows- even though the villains are supposedly different, they always seem to do the same thing each series- kill/eat people, or get a belt they can't keep their hands on. The only real standout I can think of is Isaka, who- surprise surprise- sets up his own "evil organization"-style thing to make the Leangle belt. Based on what I've seen of Hibiki, it looks like more "ancient demon/monster things out to kill people for some reason", which is okay for one or two series, but six series in a row?