Monster rider
 

In some of the KRs, some riders turned out to be an enemy's monster.

ex.
Takumi/Faiz: Wolf Orphenoch
Hajime/Chalice: Joker
Eiji/OOO: Kyoryu Greeed

But what do you think if in some series like Kiva, Double & Wizard...

Kiva: Instead of that colored glass on Emperor Kiva's visor, how about a half-breeded bat fangire form instead? King's bat fangire form looks ugly, but in Wataru Kiva's look, it should look Batman Beyond or something.

Double: If Shotaro gets under a mind control or something, what if Shotaro becomes the Joker Dopant while Phillip is forced to become the Fang Dopant to save his fallen partner?

Wizard: Haruto should've became the Dragon Phantom. Too bad it didn't happen. If it did, with Koyomi's death affecting him, he'd be drowned in despair.

I spoiler tagged those examples. Not everybody has seen those shows, no reason to ruin plot points for them.

Rumor had it that Gentaro was going to be Gemini Zodiarts originally at first then they change it to having Yui being evil instead seeing that they had the whole evil version of the main rider in OOO.

It would be interesting to see more Rider's as monsters. After all, the original Kamen Rider pretty much was in a way. He lost his humanity by becoming a Shocker experiment but took those powers and instead used them for good.

I'm disappointed they didn't bring back "the rider is a monster!" routine on Fourze & Wizard.

If it happened, they would've had...

Fourze: A zodiarts rider who is actually a part of the horoscopes. Sees the whole world as his enemy, but Gentaro breaks the barrier around him slowly.

Wizard: A mage who is actually a phantom who took on the form of his gate. But it turns out that his gate was already filled with despair but no hope. Before his dying day, the phantom swore an oath to his gate that he'll give him some hope, a happy life & purpose.

This is one of those aspects that has become synonymous with Kamen Rider. Even if the main hero doesn't become a monster in the most literal of sense, there is an argument to be made that they are one. The general idea seems to be that a Rider's abilities will almost always have a deep connection to the antagonists of the show. Kamen Rider's been pretty good about continuing the loss of humanity theme throughout the ways, it tends to tie into the idea that Riders are heroes born from suffering. (this is why Fourze is literally the only Rider without the iconic tear marks on his helmet)

Honestly, I'm fine with the thought of the Rider not BEING the monster, but rather having the same source of power. Imho, the concept of Fourze for example was good as it was and it wouldn't have worked with Gentarou being a Zodiarts. Also, about OOO Eiji wasn't a Greeed from the beginning on. He became a Greed because of the purple core medals choosing him. So in theory, he wouldn't count as a rider actually being a monster. Also, Eiji was the only Greeed who could control his desire up to a certain point, so in the end, he remained a human on the inside and never fully turned into a monster.

Actually, as for the tear marks, based on that theory, I guess Fourze would have those, if the helmet wasn't completely black. Because even though Gentarou didn't have to suffer as much as many other riders, Gentarou also had to face a lot of hardships on his way.
Besides, there's more riders who don't seem to have them then Fourze or at least they are not as recognizable? Gaim, Wizard, Decade and Den-O for example don't seem to have them or at least not as easily visible like others, despite having a good background story for those.

Yeah, I don't really buy the teardrops thing. There are quite a few riders looking at images where they aren't there.

And basically, for most riders, the teardrops are used to see through.

Gaim's are represented by the blue lines that meet up with the eyes, Double's by the black triangles at the corner, the bottom half of the silver lines on Wizard's face are his tear marks and Decade's bar codes serve as his. The tear marks are a little different in every design, mostly to remain stylish, but they're there. Fourze's producer in an interview made it a point to declare that Fourze would be the first Rider with no tear marks until his eventual power up came along.

But yeah, the tear marks most definitely were originally just a way to explain the holes in the helmets. Over the years they've sort of just evolved to become part of the iconic look of a Kamen Rider though. (especially these days when there are so many ways around having them)