Recommended syntaxes for user ids within an organization?

Jim Fournier asks about a topic which I’d like to put on the agenda for this week’s TC meeting to give recommendations if possible.

If an organization wants to give usernames to its users, what are the possibilities for naming conventions that are short, simple, and not confusable with existing online conventions, e.g. # is usually a hashtag for a topic not a person?

Some examples to start - which of the following are recommended, not recommended, or illegal?

+org*user
+org#user
+org!user
+[org]!user
+<org>.user
+org=user

Note the characters - _ . are legal as part of identifiers, so that +org-user +org_user +org.user would all be parsed as single names of organizations separate from +org and not covered by +org.