with this reveal, as well as another bit of meta knowledge thanks to the box for the GotchaTornado weapon toy, I've figured out the reasoning behind what cards get what rarities in the sets (so far):
I'll group my thoughts into sections and spoiler tag them for the sake of keeping the post smaller on screen and easier to read
first up, Meta Knowledge from earlier toys/official reveals/Phase:01 cards:
Spoiler
SteamHopper, ApppareSkebow, and AntWrestler forms were all revealed before the first episode had even aired, and are all packaged together in the first piece of the Souchaku Henshin toyline. in Phase:01 the only cards that got to be UR rarity were the cards for those forms and Golddash, which seemed an acceptable exception because it's the bike so it should get a fancy card, too.
despite the fact that those three forms were revealed way ahead of time, we got VenomMariner as a form with a proper costume before AntWrestler even appeared in the show proper, and yet those two cards didn't get to have UR versions in the set. this confused me a bit, but then they revealed they were the UR cards in the first wave of cards that would be included with Chemy Wafers, so that made sense, though I was still having trouble figuring out how the armor would reconfigure into a Wild Form, like the others, for the sake of the figure toys if nothing else.
after that they introduced BurningGorilla, another form that hadn't been pre-hyped, had armor pieces clearly reworked from previous Riders (VenomMariner's arms are both modified versions of Build's fan arm from KirinCyclone, BurningGorilla's arms are modified from Punching Kong in Zero-One), whose cards (or card in this case, since GorillaSensei is Phase:02) didn't get a UR/parallel version, and didn't look like it could easily turn into a Wild mode as a toy, which should've also been a hint when neither it nor VenomMariner used a Wild form in the show.
lastly, we just got DokkiriShovel this past weekend and it also didn't get UR/Parallels, look like it could feasibly make a Wild form as a toy, and didn't use a Wild form in the show. This form, alongside NeedleHawk, clued me in to two things: the first was the lack of Wild forms used in-show from it and the other two, because NeedleHawk DID get to use one, and also NeedleHawk got a fancy henshin sequence where they combine into armor inside the CGI flask like the first three forms, and the other three DIDN'T get a fancy sequence.
what all of those three forms DID get, though: they were the other SR rarity cards besides the main forms and GoldDash.
despite the fact that those three forms were revealed way ahead of time, we got VenomMariner as a form with a proper costume before AntWrestler even appeared in the show proper, and yet those two cards didn't get to have UR versions in the set. this confused me a bit, but then they revealed they were the UR cards in the first wave of cards that would be included with Chemy Wafers, so that made sense, though I was still having trouble figuring out how the armor would reconfigure into a Wild Form, like the others, for the sake of the figure toys if nothing else.
after that they introduced BurningGorilla, another form that hadn't been pre-hyped, had armor pieces clearly reworked from previous Riders (VenomMariner's arms are both modified versions of Build's fan arm from KirinCyclone, BurningGorilla's arms are modified from Punching Kong in Zero-One), whose cards (or card in this case, since GorillaSensei is Phase:02) didn't get a UR/parallel version, and didn't look like it could easily turn into a Wild mode as a toy, which should've also been a hint when neither it nor VenomMariner used a Wild form in the show.
lastly, we just got DokkiriShovel this past weekend and it also didn't get UR/Parallels, look like it could feasibly make a Wild form as a toy, and didn't use a Wild form in the show. This form, alongside NeedleHawk, clued me in to two things: the first was the lack of Wild forms used in-show from it and the other two, because NeedleHawk DID get to use one, and also NeedleHawk got a fancy henshin sequence where they combine into armor inside the CGI flask like the first three forms, and the other three DIDN'T get a fancy sequence.
what all of those three forms DID get, though: they were the other SR rarity cards besides the main forms and GoldDash.
Spoiler
I'd been wondering for a while why the toy for that hadn't been released when AppareSkebow debuted, since they usually release toys as close to their in-show appearance as possible, to ride the hype of the kids having just seen them get used to make them go "mom! dad! can I get that thing Kamen Rider just used to beat the bad guys, please?!". Having now seen the box on a toy review channel, I know that it's because the box contains a spoiler for an upcoming form (probably debuting either this coming weekend or the one after it), as well as having NeedleHawk prominently on the box, so that would've been TWO forms spoiled early, and they tend to try and avoid form spoilers beyond early reveals to hype up a season or in magazine articles closer to a form's debut episode coming out. the other spoiled form is, since the toy includes the Mechanichani card, which pairs with GoldDash, it shows off that GoldDash + Mechanichani gets to be a form with an actual costume.
Spoiler
as expected, HawkStar and Saboneedle are both cards that get UR versions and parallel arts, as is Mechanichani, lining up with my theory that proper forms get them, since it pairs up with GoldDash who got UR/Parallel versions in the previous set. this means, if my theory is right, we also have one more proper costumed form spoiled, since Raidenji and JungleJan pair together and are the other UR/Parallel cards in the set besides the parallel arts for the Legend Rider cards.
also: the only cards to get SR rarities besides the 5 that get URs are GorillaSensei (BurningGorilla), Venomdake (VenomMariner), and FlayRose, which pairs with Hiikescue, the only SR in Phase:01 that hasn't been used in a form yet, so currently guessing that means ANOTHER costume, but in the style of VenomMariner, BurningGorilla, and DokkiriShovel where they don't get a sequence or Wild form.
the other thing I notice, though, is that every card in Phase:02 that isn't an EX rarity has at least 2 rarities:
Legends get Legend and Legend Parallel, basic costumes get Common and SR, Proper Costumes get Rare, SR, UR, and Parallel (interesting that they DON'T get commons), and everything else (again, EXs aside) gets Rare and Common
but in Phase:01, three cards ONLY got common versions: Kaiserbee and Karyudos, who both pair with cards from the same category that hasn't been released yet, and Gekiocopter who pairs with a second category that hasn't been released yet (both categories have multiple cards in Phase:02 that pair with them it seems, so those categories will probably be in Phase:03)
I don't think that necessarily is relevant right now, unless they're saving the other rarities of those three for a future set because they're also important. that part I can only speculate on
also: the only cards to get SR rarities besides the 5 that get URs are GorillaSensei (BurningGorilla), Venomdake (VenomMariner), and FlayRose, which pairs with Hiikescue, the only SR in Phase:01 that hasn't been used in a form yet, so currently guessing that means ANOTHER costume, but in the style of VenomMariner, BurningGorilla, and DokkiriShovel where they don't get a sequence or Wild form.
the other thing I notice, though, is that every card in Phase:02 that isn't an EX rarity has at least 2 rarities:
Legends get Legend and Legend Parallel, basic costumes get Common and SR, Proper Costumes get Rare, SR, UR, and Parallel (interesting that they DON'T get commons), and everything else (again, EXs aside) gets Rare and Common
but in Phase:01, three cards ONLY got common versions: Kaiserbee and Karyudos, who both pair with cards from the same category that hasn't been released yet, and Gekiocopter who pairs with a second category that hasn't been released yet (both categories have multiple cards in Phase:02 that pair with them it seems, so those categories will probably be in Phase:03)
I don't think that necessarily is relevant right now, unless they're saving the other rarities of those three for a future set because they're also important. that part I can only speculate on
1) any card with an SR version gets to be a form with a costume
2) any card with a UR version a) gets a parallel art variant, and b) gets to be not only a costumed form, but one that's more thoroughly designed and gets all the bells and whistles
3) EX rarity (at least in the main sets) is just for the level 10 in each category
4) Legend Rider cards get alt arts just because they're special, so they're outliers and shouldn't factor into speculations on card relevancy (aside from them being in the spinoff thing coming in November)
5) anything else is probably just going to be a quick one-off attack form using either quick and cheap CGI or a drawn combination of the two parts together, like OdoriMantis or SpicVhale