I see an other solution that might work, with no heavy code work.
With the custom wizard plugin, you may do multiple actions in one procedure. So in that case, you would ask the user to write their sub-topic with a wizard that would:
- Create a new topic for the subtopic
- Create a replay in the main topic linking the subtopic.
With some custom CSS for all the main posts (you can add a CSS class to select them with a category or a tag), you can modify the replies appearance to match what you have in mind. You might also find a way with the wizard to add a parameter to the replies that will allow you to select only the automatic replies.
If you don’t like the reply solution, I think the custom wizard may also add the tag on the created sub-topic.