In the showa era this would have been pretty easy. Yeah, there were a few exceptions, such as Amazon not being a cyborg, but otherwise things were pretty constant. But in the heisei era things seem to have changed to the point that the only thing they all have in common is being superhuman. Kuuga ditched the need for an operation, Ryuki dropped the compound eyes for most of its' riders, Faiz had non-humans becoming riders, Hibiki's powers didn't in any way come from the enemy, Core wasn't even portrayed by a man in a suit...and on occasion they even turn the classic elements on their heads - look at the Oni brothers from the Onigashima Warship movie, who are not generally regarded as riders but henshin into compound-eyed armoured forms much closer to the "classic" definition than either Den-O or Decade.
But I think as far as major characters are concerned, and certainly for title characters, there are still a few points in common. Most major characters, and all title characters, will have at least three of the following four points apply to them:
A person from Japan (or of Japanese parentage) who uses an external, belt-stored device to trigger a near-instant change ("henshin") into an armoured, superhuman form to openly battle a defined enemy (nobody will attempt to use stealth or be confused as to who they must fight).
The source of their powers is somehow related to those of the main series antagonist (either using the same power source as them, one developed by them, or being directly powered by them).
They have an attitude that sets them apart from other people. Consequently, while they will have allies (ranging from a single confidant to a group of trusted friends or even a designated partner), in combat they will largely fight independently, with little in the way of combined attacks or powers requiring multiple riders to activate.
They will appear in a mediocre crossover film made only to squeeze more cash out of the fans at the expense of the long-term credibility of the franchise.
