Convert all text-parts to UTF-8?
If we would convert all incoming text-parts (or anything that has a charset) into UTF-8, we might have less trouble with the 'mail' gem. E.g. in https://github.com/mikel/mail/pull/738#issuecomment-303083226 the maintainers explain that they always prefer converting to UTF-8, regardless of what the original charset header says.
From a technical standpoint I don't see a problem changing all parts to UTF-8. I would even advocate for it, because it standardises on a modern solution.
But I can't really judge how many computers out there still don't know UTF-8 and would show broken texts.