Ultimately, I would want at least one character of ZUN's old works to actually be brought back for the canon series since it would be quite a big boom for the side of the Touhou fandom that follows the PC-98 series and the folks that are aware of it at the very least, I myself being a person who started out with the roots of the series would be one of those people who would be doing back flips at the return of those beloved characters; it would bring me great joy to see a PC-98 character rise in fanart, people making content for, and given new purpose through ZUN rather than just left in obscurity with only passing glances.
How would they be modified, in terms of both backstory and how they look? I got no idea at the moment, so I'll see what you guys could think of until I come up with something.
Eh, I'd be less concerned of backstory and appearance, but in terms of my own fantasizing and idea generating, the ideas that've swam in my head for a while is based on the fact that when it comes to an reappearance from PC-98 to Windows era is that any returning character could have a
drastic overhauling behind them.
Since it's been talked about here, take for instance a character such as Elly who lived as a gate keeper to Yuuka's mansion, Mugenkan, of which is not present in the overhaul of Yuuka as we know of now as a resident of the Garden of the Sun. If I had to guess, I'd imagine if Elly was carried over, she would still have affiliation with Yuuka given her guarding the Mugenkan
directly and likely living with her and an attendant to keeping the sunflowers of the field tended to (especially so Yuuka can travel throughout the seasons to see flowers like she does in canon) as more of an assistant to Yuuka then guardian. I know it's a bit of a downgrade role wise, but hopefully it at least fits with Ms. Kazami's rework as she is now and because at least it would make sense given the changes. Oh, and definitely her using that awesome boomerang scythe for duels would be fantastic to see in modern Touhou.
I see Elly overhauled as someone who is a more social extrovert contrast to Yuuka's now more introverted tendencies given her overblown description in Akyu's Gensokyo guides as having the "worst" relationship with humans (which Yuuka is hinted to have influence over likely as a means to keep humans away), but I'd imagine Elly would do that to draw human and youkai attention away from Yuuka (given how Yuuka avoids the humans in PoFV, scares fairies, and 'teases' others).
Admittedly any ideas I would have of Kurumi is not something I have any assurance of canon-based ideas like Elly since it's ambiguous if she was a gate keeper
for Yuuka's mansion or just the realm it was found in, but if there was anything I could come up with, it's that Kurumi also gave up gate keeping and inherited Yuuka's mansion for herself since it'd be vampire-like owning a mansion of her own, and it'd make good use of the idea of making use of the idea behind Mugenkan in a canon setting even if the idea isn't based around actual canon, I don't know, I like the idea of Kurumi getting a glorified mansion is all.
I cannot speak of unique appearances with too much to back since I'd have to rely on specifically drawn fan art for ideas, at least for Elly I have
this Pixiv picture based on HM's sprite art style, but cosmetic design is likely secondary here since most of the returning characters from PC-98 do keep their appearances at least reasonably close to the PC-98 counter part, so I'd mostly focus on the overhaul piece if anything, but I'd like to know what you think of this, Colonel Summers.