I like the general shape of the changes to this feature as suggested by mark, though I think I'm leaning mildly more towards the mastodon-style implementation matgb mentioned of @user@instance.tld, mostly because my sense is that more places let you have "." in usernames than "@". (Instagram, for example, definitely permits periods in proper usernames; I would be concerned about how to properly link to some usernames there using the proposed shorthand.)