It is not uncommon that in answers or comments one refers to some other user. I’ve seen a lot of answers which for example start with “The answer [username] gave is great, but I’d like to add a different approach” etc.
Now I’ve observed that in a lot of older questions those usernames that were referred to where nowhere to be found in that question. This has of course to do with the fact that users can change their username. And if questions are older it is not unlikely that a user which has been referred to in an answer or comment has changed his username in the meantime. Often you can figure out who was meant if for example there are only two answers or by examining the time stamps of answers etc. But other times you can’t.
This is where username tags come in. Analogously to the chat, wouldn’t it be possible to implement tagging a user via @[username] in comments and answers and let the system display it as the plain text username? One could implement it so that when the tag is made it is linked to the unique user ID which is associated to the [username] at the time of the tagging and therefore the tag is independent of a change of the user name. To prevent tagging from escalating, one could limit the usernames which are allowed to be tagged to the current question i.e. to only the users which have participated in the question by writing an answer or a comment.
I’m aware that the tagging is already possible in comments, but I believe it doesn’t adapt if the user name changes and after all it is not possible yet in answers.