I’ve had a while to ponder this further, and I’d like to add a bit of clarification.
I’d like for the category to be ‘pre-populated’ depending on what topic the user is already in.
Here is an example of a Discourse forum where this is happening:
So if the user is in the Underground category and they press ‘Create Topic’, ‘Underground’ will be pre-populated in the Create Topic dialogue. And if they were in the Otherground category, then THAT would be pre-populated.