This is exciting to see Michael, and I echo Matt’s appreciation of your commitment, consideration, and patience. Software development and community management are both imperfect, messy things, and when you smash them together (as is necessary in a software tool community 
), it can get even more so.
I think for now you need to keep them separate and use Fibery relations internally to associate public feature requests with internal features, product areas, etc. And I’m thinking about it and not actually sure how it could be otherwise.
Let’s say you have a perfect Fibery 2.0+ in the future that has the ideal permission model to show your “roadmap” to the public and let people add requests and vote on everything. Do you think most users are going to make appropriate decisions about how/where to put their feature request in your internal organization of everything? For example I would be willing to bet nobody would have ever put anything into “Fear management”, because what the heck is that? We in the forums know what it is because you’ve explained it many times here, but the average person wanting to make a feature request never would. I would bet there are plenty of other potential examples.
Of course you could just have all Feature Requests go into a single “uncategorized” type of place and then Fibery team manually moves it where they think it goes, that could work. It is not much different (in my view) from connecting Discourse topics with internal Fibery tracking, although it is a bit more complicated, less integrated, and not bi-directional as you say. But it seems that manual work is necessary to manage it in either case.
So some improvement could be made, but my point basically is that the issues you outline of Product Area, Idea, etc. complexity are not unique to Discourse and I’m not sure you’d want to map that model to Discourse nor fully expose it directly through Fibery even if you could.
This to me is super exciting because I honestly have not seen a really great product in this space! Productboard is surprisingly bad, in my view (no conversation, no participation between users, just isolated feature requests and a form of voting). Canny is the best one I’ve seen so far, at least for the front-end/user experience, but it too has some issues, and more importantly it lacks in integrations with most actual product and dev management tools except through e.g. Integromat. And if it’s all implemented right, i.e. in a flexible model as Fibery has been so far, then it should be useful for lots of other things I would think. Public form view and submission, and some way to handle “public” (non-user) interactions for voting and comments, ways to create public-“sanitized” or appropriately limited views for e.g. public roadmap without exposing all internal discussion and details, etc.