Apologies for my late reply.
Opening the composer (in a blank state) from an URL would be useful for those who have external websites working with Discourse and ask their users to reply to a particular Discourse topic.
For instance, I have a private website where I teach online lessons. I ask my students to reply to specific discourse topics to introduce themselves, to post their homework, etc.
It would be nice to be able to add something like
?reply at the end of any topic’s URL, and share that link with them.
It would open a new reply to that topic with the composer open.