I've run into this trope before and it's always baffled the heck out of me; I think it migrated here from Japanese fandom and...well, I'm trying to be charitable and assume it made more sense in its original setting? Best I can guess is that maybe it was originally a comic trope, because it could....kinda (?) work in a really visually spare, shoujo-y oneshot comic with minimal worldbuilding. But for a fanfic au setup it breaks down. Please deconstruct the heck out of it.
And it's common fanon nation-tans can't be killed while their countries stand, so the body horror could be played up because they'll be choking up flowers for ages and possibly forever and never die from it... That was my gripe about certain infamous gore fics in the fandom, BTW. Why is it so bad that a character got his arm cut off when it's shown in canon they can survive falling from planes and randomly dropping their internal organs? Going by the way they work in so many other fanfics it should just grow back!
this reminds me of the undertale au, flowerfell, in which flowers grow on frisk's body every time frisk resets.
Im imagining like, haunted forests where people with the disease go to die and then turn into a tree and exist with all the other unrequited treeople. Just an entire forest of sorrow and longing. You can feel it like a heavy fog around you
...deep in my terrible heart I want all the body horror fics for this, now? Also, while I Do Not Go to Hetalia, if I were gonna approach the hanahaki idea, if you had two countries who were politically at war but one of them was in love with the other, it would in fact be strategic for the object of affection's ally countries and/or human leaders to keep any and all knowledge of the enemy country's feelings from the object of said love for as long as possible, so as to prevent any likelihood of those feelings being requited. Like, you'd work it into the strategy, wouldn't you?
Other more accurate thing I'm reminded of by the forest thing; the Wood of Suicides in the first Sandman book.
Probably otgw too, im realizing. Would "coughing up plants" verse have a woodsman type character, or maybe an in universe fairy tale about a reaper-like character who uses the trees and flowers to make more of the disease? Could be like what people believed before modern medicine. Maybe the disease is from the trees themselves, like the pollen and seedpods or whatever contain the stuff that makes people get the disease so more trees can be made so theres like, whole quarantined forests and fields where idiots do tests of courage and the like because the plants themselves have an unearthly feel to them and then they get the sickness. Standard procedure is to burn contaminated corpses before they bloom, and it (obviously) has a super high mortality rate.
Well I think both of those were inspired by Dante's Inferno, where suicides took the form of trees after death.
Getting even darker, what if there were like, black market collectors who had greenhouses of love disease plants.
I can sort of see how this disease would work if there was a scale of effect; like, fictional character crushes make you maybe sneeze a couple of petals and that's it forever, and the actual lung-filling stuff only happens in really devastating circumstances. And the aesthetics are super cool in a body horror sort of way. Still weird and unfortunate implications. Apparently this manga is the source of the whole business. Thank you, Tumblr!