Harmony

I'm currently working on Harmony Forum which is a native Drupal forum built with two entities (Thread and Post) with Taxonomy for categories should you want them.

That's the shortest possible description which outlines an entity based forum, a bit more in depth would be leveraging contrib as much as possible to produce something like Discourse (http://try.discourse.org). All whilst coding so that Views are used and can be overridden, display elements can be changed with Field UI and that there are plenty of hooks and chances to jump in and do what you need to.