I think its a great idea. Topics shouldn't be the same as those found on hackernews, those would be redundant though. I would suggest using discourse.org instead of rolling your own phpBB instance
I'm honestly not the biggest fan of discourse, though I do appreciate it as a project. I like the old school BB style.
One opportunity we have as a forum is to create continuity between the trending conversations and more "long term" discussions that are difficult to have on a platform that is churning out new content constantly and lacks notifications about replies etc