I think we should do whatever !xmpp
does with usernames. There are many lessons to be learned there (unicode treachery etc.)
Also, yes I think URLs, as it is in !GNUsocial
already, should be possible to mention. Not just acct: style URIs.
I think it's important to remember that some users may want just their domain name as identifier for example, which btw makes parsing/prioritising a bit trickier if usernames can have dots in them ;) But I think there are solutions.
And on that topic, usernames can be @ but don't support '.' in GS. If supported we'd probably prefer the local username if not appended by '/'.