Could do with a Poetry Ribbon for sure, and maybe Rain Man since that would be a reference-within-a-reference. Maybe rain man adds 3 poetry ribbons, 3 plain ribbons, or 3 blue ribbons to your deck, all of which have value ?3? and are Hearts, Diamonds, and Spades respectively. Then the ribbons say if you have three ribbons in play after playing one you can choose to force you and your opponent to stand immediately or not do that (but hopefully as an on-play effect, not as an on-click effect, so it better reflects calling koi-koi). Maybe they should be value 5 and give blackjack on Akatan/Aotan idk. Something like that would definitely be fun.
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Rain Man (0):
On Play: Choose to add a Poetry Ribbon or a Blue Ribbon to your draw, play, and discard piles, then burn this card.
Ribbon (7):
On Play: If you have two ribbons of the same type already in play, move all other cards in play to owner's discard pile. You and opponent are each forced to stand, and your bust limit is set to 21.
It's not bad, but I think the ribbons should be worth less, since they aren't doing anything useful mechanically and bigger numbers are worse. I got 3 because it's smaller than 5 but a factor of 21-- which is I assume how you got 7-- and 3 and 7 are equally close to 5 (the value of the card in most hanafuda games that care about card values) so that's a wash I think. Also I don't think Rain Man needs to burn on discard-- adding more ribbons isn't too OP I think, especially if the kind that's added is random. And if they are 7s its actively bad for you.
Also yours would make you bust if you have any locked non-0 card, which might be a little problematic.
I said "move to owner's discard pile" as a way to circumvent locked cards without complicated phrasing, because the text is long enough already. This would also use the interaction where locked cards that are moved somewhere other than the play pile remain locked.
If the ribbons were worth 2 instead of 3, then to create a blackjack with 3 ribbons each one could gain 5 value and then reset later. (In addition to being the value of the cards, 5 is also the value of the Akatan / Aotan combo.) I was actually thinking the different types of ribbons could have different functions. Poetry Ribbon could be another all-suits-at-once card, while Blue Ribbon could be spades or nothings and have an additional blackjack effect.