Haha.. that's funny you say that, because one of the primary reasons I built a dashboard for SS3 was to break away from the antiquated and annoying site tree paradigm! I was actually really disappointed to what a strong presence the site tree continues to play in SilverStripe 3. I can't stand that thing.
So needless to say, it's not something I'm going to add on my own, but if a pull request comes along, I'd certainly entertain making it an option. I think you're right that the site tree is modular. That was one of the primary goals of SS3 with respect to the JavaScript layer was to make all these widgets controller and template agnostic, so I'm sure it could be done easily.
Another idea would be to create a Site Tree dashboard panel that shows your entire tree, with some fancy AJAX behaviors for loading children like the site tree does now. I'd much rather do that, I think. Each panel is a full stack MVC, so all the tools are certainly there to do it.
Hmm... now I'm tempted to do it.
UC