1 sub reddit is only for one problem. Inside 1 sub reddit, there will be multiple “ideas”/topic on how to tackle the problem.
The use case is actually more like Facebook comment/Disqus plug in which can be attached to a page, but with much better way for discussion. The problem with those two plug ins (especially Facebook comment): people write comment, leave, and seldom come back. While I think Discourse is more suitable for idea sharing and discussion.