If possible, it would be really cool to have custom fonts (stored with the user, maybe? idk a lot about the practical issues at hand) so that you could make posts with any font you'd like without the viewer of the post having to have that font downloaded to see the posts properly. Personally, i'm pretty into constructed languages, so it would be very cool to (for example) post in a custom font for a script that isn't in unicode (could be JRR Tolkien's tengwar, the sitelen pona script for toki pona, or even something i designed myself). This is just what i'd want to use it for, there are definitely a lot of other use cases too.
If this is done I hope it's compatible with screen readers. Additionally, there'd be a way to turn it off for those who have poor eyesight but no screen readers. I recall this being an issue with tumblr and their 'fancy text' options.
I don't mean to naysay off the bat, this sounds cool and I'm sure some CSS crime posts would benefit from fancy font stuff too.