Discourse won’t be for issue tracking – it would be for issue reporting. Users can report these in the existing User category. This way we can figure out if this issue is worth working on (and then put it in Trello), or it’s just a question etc.
Trello is for task / issue tracking. If we choose to continue using Github Issues for issue tracking, then we’d still need a way to synchronize between the two.
Trello for Roadmap, not a bad idea. I see this one: Trello
Update: Along the lines of what Pawan mentioned, Zapier uses Github Issues for bugs only. They also use Trello for product planning and features.