@Thomas_Shafee suggests being able to use the preload functionality in the Reply tool. This could be useful when you need to have a structured reply to a question. See this comment on MediaWiki.org.
Use case(s) (describe the actual underlying problem which you want to solve, and not only a solution):
Thanks! In general, it'll be useful to be able to control both relies and new discussions in similar ways, especially via the URL. The use case on this page is probably extreme, but there are plenty of pages where replies expect to have templates in (or around) them. The most common is probably voting, where it'd be useful to have three buttons support/oppose/comment that add a reply with one of those templates (similar to Template:Vote_below).