So I looked into customizing this more — and it seems the builtin discourse customizations available to me are indeed limited to a single line: New lines in the composer's reply placeholder? - feature - Discourse Meta
Now, that said, as pointed to me in the above thread, we could provide topic templates. This would act like GitHub issue templates — it’d just be prepopulated text in the compose window (instead of a shadowed placeholder). Or we could do some theme hacking and try to get a placeholder like the R one above.
One of the nice things about topic templates is that they just appear on creating a new topic — and they can be customized based on the selected category.