RX's monster designer was Keita Amemiya, who had just recently worked on Metalder and would go on to do Jetman in a few years. and, of course, he would go on to create Garo about fifteen years later. If you watch Metalder and Jetman you'll notice that they have very similar monster aesthetics to Black RX.

I know is too early but, how would look like the riders in the 2020's ?

Not counting Organic Riders who's bodies become their 'suits' like Another Agito, how do you think Riders see out of their helmets? Sentai Visors have limited vision but are somewhat larger and allow for greater peripheral vision, I think, than Rider Helmets. Or do you think they're like what we saw with Birth? Tech readouts and things. Especially with helmets that have compound eyes and such, Now Gaim got away with this. Almost every helmet was a visor style in one way or another, but there were exceptions. Like Marika. Personally, I think the helmet with the best vision would've been Wizard. Basically his entire helmet in every form was the visor. Like wearing a fishbowl. Kamen Rider Mysterio. The worse? Fourze. I think. Eye holes plus fin ears. So even if he had peripheral vision he wouldn't be able to see past the fin ears on the sides of his head.

Decade. The eyes aren't actually eyes for the helmet iirc, but rather the bar thingies. They showed it in a Net movie

I chalk Decade being able to see at all up to him being a massive troll.

I think we have three categories here: Cyborgs, suit wearers, and magic / supernatural beings.

Cyborgs, like most of the Showa Riders, have greatly improved vision through their upgraded bodies so they can see better than a normal human in a suit. Even though the Riders from Blade mostly fall into the suit wearing category I've seen a POV shot were a Rider sees in a honeycomb pattern and can zoom into each hexagon individually. That's kinda how I assume the cyborgs with the bug eyes see, only better.

Suit wearers are like you described Birth, they have visors and computer readouts to aid their vision.

Magic / Supernatural are Riders who transform via magic or out-worldly powers, those I assume just see normally as if the helmet wasn't there and have special powers depending on how their abilities aid them. Because magic. It might also be a mixture of both cyborg-like and mechanical vision.

Perfect. Beautiful. *claps*

To be fair, I must add that we can't sort every Rider from every show into a category, the Blade Rider I mentioned doesn't even have bug eyes and he wears a suit yet he doesn't have an overly techy vision.

I guess it's just that we can assume they see like they do in certain cases since Toei obviously isn't giving us the answers we need, POV shots from inside the helmet are rather rare.

Then there's tech Riders like W and Drive whose bodies seem to actually change, where I would imagine their "helmet" is actually just their head.

With Double I understand what you mean but why Drive? I could have forgotten but I don't remember how his body changes exactly.