I should probably look up how these events and decisions work because this seems like something that should be added to the Fairy Alliance civ, too. Probably mirror images or precursors of the Marisa ones. Fairies get "Go Cause Trouble In The Forest of Magic?", Marisa gets "Fairies Cause Trouble! What do?" Fairies get "Challenge Ordinary Human Magician?", Marisa gets "Accept Challenge From Fairy?"
The Events and Decisions mod is quite incredible, and has been a lot of fun to incorporate into the civs I'm working on. It's been a while, so I can't recall how familiar you are with Lua, but once you've got a hang of the basics of how the events and decisions code works, it's incredibly easy to add new ones, and they're extremely flexible.
Currently, Marisa has an event -- "Fairies Cause Trouble" -- which has generic fairies causing trouble in the Forest of Magic by playing pranks on people. It's loosely based on the way the Three Fairies of Light would play pranks on Marisa, but over time grew to be more comfortable with her instead. So if you ignore the fairies, they'll eventually reveal a Fairy Tree (a unique improvement) in your territory.
Or you can just send out the troops to drive them away. Or blast them with lasers!
From what I know of the code, though, it
would be possible to have an event occur for Marisa after Cirno declared a Fairy War, in which she got challenged by a particularly determined fairy who declared herself "the strongest"...
hmmm... I wonder how doable some of these ideas are. I like the idea of making it less "a choice IF you want to use it" and more "a choice WHEN you want to use it." That idea gave me this idea for a Fairy Alliance decision:
Begin the Great Fairy War!
You have decided that now's the time. Time to show the big people of Gensokyo what fairies can really do! By rallying your armies with impressive speeches and colorful drawings of youkai and shrine maidens with X'es for eyes, fairykind unites on a grand crusade to carve their name into the tree stump of history! No longer will you have to spend all your money keeping everyone focused on their jobs -- but be careful, because if the grand crusade you declared fails, it's gonna be really, REALLY hard to get fairies to take up arms in your name again!
This decision would have little to no prerequisites, and permanently flip your abilities inside-out: halved unit upkeep costs, halved unit production speed. Maybe give all existing units March, because I feel like halved costs and production is a lot less attractive than the other way around. The idea behind it is you're saying "Okay, I built my army, I think I can conquer the world with this if I don't have to keep paying through the nose for it." It's not a decision of what cultural path to go down, it's like the decision of when to use your warcaster's feat in Warmachine.
Okuu would have something conceptually similar, "I'm ready to cut all ties with everyone and transform the planet into an irradiated nuclear hellscape out of love." What would Eirin get? Something about hiding herself or hiding the Moon or both?
Warmachine, hmm?
It is possible to make decisions that act like toggles -- the Egyptians have a pair of decisions that allow you to determine whether you get gold or faith from passing other Decisions. The main issue would be whether or not you could flip those effects with Lua -- I can't remember exactly how you implemented Cirno's trait, although I think it was through dummy buildings, in which case you could simply swap around which dummy buildings were present in the city. (Although I don't know if Cirno's UA would then try to override this...?)
I think the Lunar Capital Expo would be a great decision. I'm not exactly sure what it'd do, but given Eirin's emphasis on GPs, something like "gain +1 Tourism/Culture/Gold for each Great Person improvement in your territory" or something like that might be good.
Speaking of Kosuzu, one decision I thought of for the Tanuki was a reference to when Mamizou gave Kosuzu a book to encourage people to be nicer to the tanuki. It was something I didn't get to include in the core civ, but the decisions are a great way to incorporate it. It'll most likely help you gain favor with city states, although possibly at the expense of a great writer. (Or it may simply tie into how many great works of writing you have?)
It just goes to show how awesome the decisions system is -- it's a great way to include all of these little tidbits about the characters that couldn't really get included in the civ as a whole without making them too schizophrenic.