Not sure if you're on 2.3 or 3.0, but this is pretty much exactly what IP.Board 3 is ;)
In IP.Board 3, the forums application is just that, an application of a framework. You can easily change the default application to IP.Blog if you want a blog as your homepage, or to IP.Content if you want to create your own (you have to do this via an edit to the initdata.php file since it needs to be loaded before anything else, but it's just a one line edit).
You can also reorder the tabs quite easily by dragging and dropping the order of the applications in the manage applications page - true, it won't let you move non-default applications above default ones, but that's just an interface thing - if you really want any tab to be at the front, you can edit the order value in the database.
You should have a delve around in the 3.0 code - you'll notice that the bulk of the code is in /admin/applications/* and /admin/applications_addon/* which are the application code resides - one of them is forums, one is members, etc... it's pretty much exactly what you're suggesting :)