To have different forums is not always an ideal solution, because one topic could go with more than one forum. With the new thread tagging feature, categories become obsolete in some cases. I am thinking about how to use Xenforo without traditional forums, only with tags for categorization. How could I get rid of the forums? Thank you very much for your innovative ideas!
Pretty sure that would still get rid of the forum aspect of things, tags/prefixes included.
You could just use a single node for all your threads, use tags and prefixes for sorting options. Finally set your home in the options to that node. It'd require quite a bit of modifications overall to make sure experience is nice for your users but that's a general start. I started something like it for one of my forums awhile back and it started to come out nicely.
Speaking of XenForo, I would just keep it as "Categories" at the left-hand Sidebar (same as at the RM) and keep the "Tags" as they already are.
If you completely skip the "Categories", then you loose certain hierarchies (I guess some people call it taxonomy) which is not always great.