I've been reading some old Muffin Miscellany on PC-98 references in Unidentified Fantastic Object: dizzy.pestermom.com/?p=pc98ufo
And it got me thinking, the main problem with PC-98 isn't that the characters doesn't return. 90% of windows characters don't return. The problem is canonicity. PC-98 characters don't have a place in modern Touhou, because we aren't sure if they exist in modern Touhou. And since shoehorning old characters into new games is bad practice, it's a vicious cycle of characters cannot return because they would be jarring, and if they return they would be jarring.
This is the biggest problem PC-98 fans face. The rest of the fandom don't consider them canon or important enough to use them, and when you use them they stick out like a sore thumb. You are stuck a minority, and being a minority is not fun.
So how can we fix this? Like I said, the problem isn't characters returning, it's character canonicity. Nobody cares about the whereabouts of Rumia or Letty or Shizuha or Parsee when they don't return in a new game, because we all know they are hanging out somewhere in Gensokyo. We are not worried about their existence. PC-98s on the other hand never appears as background cameos or mentioned in passing, and fans are genuinely concerned that their favorite characters no longer exists. And when you're filled with emotions, it's easy to lash out in anger and disappointment every time Mima is missing. But we can fix this.
Since shoehorning characters into new games will ruin it, all the old characters had to do was appear in a supplementary material. Like I said, the problem wasn't character returning, it's character canonicity. A Perfect Memento in Strict Sense styled book where both PC-98s characters and windows characters and backstory characters and important never-appearing-in-a-game characters are explained would make the PC-98 canon and fix any contradictions. Cameos in mangas and books would also work. These characters don't have to explain where they've been, because they've always hanging out in Gensokyo like everyone else. You don't expect Rumia or Chen or Hina to explain where they were if they appear in a comic, and neither should you expect the same with PC-98s. And since the manga introduces new unimportant characters all the time like the ramen shop guy or the evil snake dragon or the tube fox or the fortune teller, the manga could "introduce" a PC-98 and it wouldn't be out of place. And you don't have to reintroduce every single character to make them canon. Simply introducing an old character would immediately make all of their relationships relevant too, making other unintroduced characters canon by association.
I know ZUN probably will never read this, because this is written in English. And also I'm not famous enough. But the point is that if you are a bit more creative, you don't have to force old characters back. And I guess the best way as fans to bring old characters back, is to keep using them in fan works. The more we use them, the less weird it would be to use them, and these old characters would be considered normal characters. Do the Touhou Puppet Dance Performance, and treat PC-98 as part of the family, because after all regardless of platforms all games are still part of the same series.