I’d like to see full unicode usernames. However note that doing this correctly is tricky and can have security implications. One needs to take care with canonicalization (same character, encoded different ways) and there is potential for user confusion (different characters, but look the same).
I.e. “Α” looks very similar to “A” but is not the same from a machine point of view (one is LATIN CAPITAL LETTER A and the other GREEK CAPITAL LETTER ALPHA). Are you talking to Adam or Αdam? Are you sure? 