Unicode handles that work with every fediverse platform today? I mean like right now...
IDN-encode (punycode) them. Been doing this since May 2018. They work with anything, even Mastodon. Any project should be able to implement properly displaying unicode handles in about 20 minutes.
The only hard part is implementing nicknames for your contacts/connections so you can auto-complete mentions of these folks using a US-ASCII keyboard.
Please share this and don't let Mastodon come up with a way to make this any harder or something that only works with their implementation.