In a scenario where Spike, Princess Luna and Princess Celestia and/or any of the Mane 6 would get to use Kamen Rider forms, Who would get which ones?
I can see Kamen Rider Cronus and Kamen Rider Woz being one of the Kamen Rider forms Spike can get due to their color schemes and such. Can't stop imagining that. And him getting Kamen Rider Ryuki too.

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Woz, maybe given he's basically Twilight's herald at times. Cronus is a bit of a stretch. OOOs would be a better choice, given dragons and desire.

Luna? Dark Kiva. Flutters would be regular Kiva.

Twilight would so obviously be Build or Wizard.

AJ would be Armored Rider Idunn.

Celestia? Possibly Agito. Possibly. I don't think she actually needs the armor to kick ass so...

Rainbow? Hmm, I'd like to say Kabuto but it's a bit obvious, isn't it? One of the Kabuto Riders, Gattack maybe?

Rarity could be Wizard, if Twilight's Build. So many gems.

Pinkie? ...yeah, I got nothing. One of the Hibiki Riders possibly?

Starlight would be Cronus or Zi-O.

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I forgot to mention that I can see one of Luna's form being one of the Kamen Riders from Kamen Rider Ex-Aid too due to the whole Gamer Luna fanon.

Hmm well I can see pinkie as either Kamen rider double with someone else or fourze due to her personality. I'm leaning towards fourze more as well pinkie likes making new friends after all.

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I'm gonna go a bit overboard and do a few more characters then you asked for. This is something I think about a lot, shockingly enough, so I have some ideas.

Twilight Sparkle: Kamen Rider Build.
This is an obvious one if you ask me. One of Twilight's most definitive features is her vast intellect: in literature, magic, and even a bit of science. We saw it mostly in the early seasons, but she was a bit of a tinker-er with a few devices to her name. Like the thing she used to try and study Pinkie's Pinkie sense. Or, more recently, the portal device featured in the Equestria Girls features. Thus it would make sense to me that she'd become Build. Which also leads up to my next point stupendously well.

Spike: Kamen Rider Cross-Z
You can't have a Build without a Cross-Z. And Spike fills that roll shockingly well, even without the Dragon motif in mind. While not as smart as Twilight/Build, Spike/Cross-Z means well and fights for the same cause. Is it a bit obvious? Yes, but it checks out.

Raibow Dash: Kamen Rider Mach.
This one is another obvious fit. Dash is all about two things: Going fast and showing off while going fast. Mach is a Rider that does those two things very well. Drive would also work if you want her to be a Main-Rider, but I feel like Mach, in particular, fits RD's personality just a bit more. Besides, who said we only had to use title-series Riders here?

Rarity: Kamen Rider Wizard.
At first, I only paired these two up for cosmetic reasons. Wizard is a very stylish suit that focuses heavy o gemstones, which is something Rarity would be all about. But upon further examination, "saving people from despair" is something that could be called an act of generosity, if you look at it from the right angle. Plus, Wizard always just lets the people he helps keep the Engauge Rings, even though they'd be useful in helping other people. A generous reminder of hope, if you will. I dunno, I feel like it works.

Pinkie Pie: Kamen Rider Kuuga OR Kamen Rider Ex-Aid.
Now, this is one I'm having a hard time with. In terms of aesthetics, Ex-Aid's bright and colorful forms fit Pinkie very well. It's a very playful power set all things considered. Plus, you could make the Orange-Half of the Mighty Bros. XX be representative of Pinkamena, as in the show the Orange Half was the more agro of the two (before Pallad came into the picture, that is). But at the same time, Kuuga's unending resolve to make everyone smile litterally matches up with Pinkie's prime directive in an almost poetic way. Both have valid reasons for fitting up with Pinkie.

Fluttershy: Kamen Rider OOO OR Kamen Rider Kiva
Again, another choice. With her love of animals, OOO was an obvious choice. But Kiva, shockingly enough, fits her personality better. Kiva was a very timid Rider who slowly came more and more out of his shell as his series went on, and that's literally just Fluttershy's character development. Plus, Flutterbat reference.

Applejack: Kamen Rider Gaim OR Kamen Rider Kuuga.
Applejack is the one who's the hardest to pin down for me. Gaim is a given because of the fruit motif, but the whole Samurai thing doesn't really fit. In the event Pinkie doesn't end up getting Kuuga, I feel like his streight-forwardness in terms of powers and abilities would be right up AJ's alley. Sorry, this just isn't one I've put much thought into.

Princess Celestia: Kamen Rider Black/Black RX
Yup. A Showa Rider. The later seasons in the show very much tried to play up the idea of the old generation (Celestia) passing the torch down to the new. And what better way to represent that then with the last Showa Era Kamen Rider (not counting the movies)? Plus, Black is constantly referred to as "Black Sun" in his first series, and as the "Child of the Sun" in RX. Combine that with the fact that he's literally solar powered in RX, and it all just comes together.

Princess Luna: Shadow Moon
While not technically a Kamen Rider, we can bend the rules a bit for the sake of aesthetics. Not only does Shadow Moon have "Moon" in his name, but he's also the brother of Black (or half brother, semantics) and was corrupted by dark forces bringing the two to fight (kind of like Nightmare Moon). And while Shadow Moon was never given any form of redemption, well, again we're kind of playing hard and fast with the rules on this one. But if you need someone who's officially a Kamen Rider... then I dunno, Super-1? Because of the Astronaut Motif (yes, Super-1 was an astronaut... kind of. Look it up).

Alternative Assignments for Twilight, Spike, Celestia, and Luna.

In your post, you mentioned how it would make sense for Spike to be Woz, due to how the color schemes match up. Well, if that's the case, then we have to match everyone else accordingly. As you can kind of take the "passing on the touch" motif in a slightly different dirrection with these Rider placements. With a few extras I threw in for fun.
Twilight Sparkle -- Zi-O
Spike -- Woz
Celestia -- Decade
Luna -- Diend
Sunset Shimmer -- Geiz
Starlight Glimmer -- Geiz
Trixie -- Tsukoyomi

The placements here shouldn't really surprise anyone, thanks to SPike's assignment as Woz. But why are their 2 Geizs? And why is Trixie Tsukomyomi? Here's why:

1 -- Geiz is a good character. As are both Starlight and Sunset. All three started off as antagonists, but eventually came around. Starlight fits here because her redemption arc involved her using Time Travel to try and undo her adversary, and Sunset Shimmer matches because of the Color Scheme. Plus, as much as I hate to admit it, but Sunset and Starlight both fulfill similar roles. If you want them to Co-Exist: just give Sunset Geiz Majesty and Starlight Geiz Revive. It wouldn't be the first time a Rider has been used by two different users at the same time. See Kamen Rider Birth, or Build's Dad from Kamen Rider Build.

2 -- Tsukuyomi is not a good character. She existed only to be Geiz's sidekick. And while Trixie is a good character, she was eventually made into Starlight's sidekick, for lack of a better term. So I begrudgingly had to match these two up.

Anyway... that's all from me. Hope this helps out in some way or another.

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Those are all pretty interesting.
Wonder what other Kamen Rider forms Spike could take?
I keep imagining Cronus being one of the forms he could use too, like I can't stop imagining that. Aside from the color scheme, theres other stuff that fed into that thought. Like how that form can use the Gashacon Weapons of the other fellow Riders, like I imagine Spike using ones based off of the Mane 6 and such. And also how Spike was involved in time-travel antics due to Starlight during the season 5 finale. Along using that Crimson Exploding Dragon Sword(though he needed to be in Gamedeus form for that, but still).
And the fact anyone can get the potential to become Cronus if they collect all the trophies and beaten all the Bugsters in Kamen Rider Chronicle(well.. that was the IDEA there till sadly there's a catch) I mean, with how sadly Spike keeps getting left out or unable to do anything due to not being an Element Bearer, him pulling out one of the most powerful(and dangerous) Kamen Riders would leave everyone struck off-guard there.
I mean, imagine how cool it would be with Spike just walking up while that huge clock in the background that was around when Masamune Dan was around, going "And judgment has now arrived" and Henshin into Kamen Rider Cronus infront of the bad guys, leaving them all shocked that a baby dragon like Spike(that sadly doesn't have anything to beat the villains that the girls do) pulling out that.
Heck, imagine if he combines Cronus and Woz into something like Kamen Rider Cronus Woz. That would surely roll of the tongue good, especially with that announcer voice that Kamen Rider Woz has.
So yeah.. I kept imagining all of that and such.

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I don't think Chronos works well for Spike. It's too sinister of a power set.

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Well the good guys tried to use it anyway with Taiga tried to use it(from what I saw on clips and read)
Too bad those 2 times didn't bold well for him, at least it kicked butt though(especially at the second time)
Besides, any power can be very terrifying/sinister if its used in the wrong hands. And even if it is too sinister anyway, I don't think it would stop Spike from getting to use it. I mean he's often got the short end of the stick, him pulling the surprise of using something like that would get the drop on anyone, especially if one's underestimating him.
And well.. I have the habit of enjoying seeing good guys using dark powers and such and being able to use them for the side of good.
So I kept having those thoughts of him using Cronus.

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This is something I've thought a lot about (at least in relation to Equestria Girls), and I've got a lot of ideas-!

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Oh. Well, someone beat me to the punch. XP

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What would your ideas be? :0

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Kamen-Zero beat me to the punch on some of 'em. Kuuga Pinkie, for example. Though I tend not to use Spike, as I use the Humanland counterparts. If you want a few more:

I would argue that Rarity would make a good contender for OOO, if not Fluttershy, because of her "Element of Generosity" status. Now, I know that Rarity can and has been prone to her own bouts of greed, but I'm talking about the magic of the artifact itself. If we take into account the magic of the artifact known as the Element of Generosity, which has taken residence within Rarity (and presumably her human counterpart), would she not be immune to the corruptive side of the magic of the O Medals and Cell Medals? That being, wouldn't it happen that she would not fall into the same trap as King OOO? Just food for thought.

For Double, I think Twilight and Applejack would make a good combo. Twilight has been known to share Philip's love of research (even if it's not to the same extremes. ...sometimes), and AJ and Shotaro are both hardy individuals good at drawing their bookworms back down to earth. Plus, they both wear hats.

Decade is, obviously, Pinkie Pie. Who else could it be?

If anyone is gonna be Fourze, it's gonna be one of the CMC; though I would argue that Scootaloo fits best, personality-wise.

Fluttershy could straight up be swapped out with Wataru with zero changes and absolutely nothing would change about Kiva.

I had to think long and hard about Faiz, but I think Moondancer could fit, as both have been known to prefer solitude to the company of others.

And that's just a few. I have a ton more, but I've got a huge project in the works that uses all these ideas. If it ever sees the light of day, I gotta save some surprises. ;P

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So pretty much Spike gets left out due to the writers making him a dog?
Well that stinks..

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And, honestly, I can’t see any Rider he could fit that one of the girls couldn’t.

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Well.. it bums me out that Spike is ultimately going to get left out during that project then..