I don't like any of the other-site syntax options, because every option I can think of is ambiguous. Some sites allow dots/periods/at signs/anything in their usernames, to which the only correct solution is escaping, which ime end users hate. I think it should be just Dreamwidth; the other sites can have their specialized escaping and formatting done by the rich editor.

Things that are pasted in should not have their ats converted, which fixes most links. (Users who both link to Medium/etc and have to retype their URLs by hand can go to settings for a fix; that is an unusual use case.) Posts made in the past should not automatically be converted. No notification should be made to the user who was linked to, regardless of whether the user sending and/or the user receiving expresses consent; the comments system works fine for public conversations.