A Taskpaper dashboard would be a big benefit so that I can see all of my Taskpaper files in a file pain, then open a file and work on it in the note pane. So this might be a three pane taskpaper. Text wrangler opens a folder in a way like I am describing but it is not a taskpaper app.
While it may be an interesting idea in theory, I’m sure it’ll be quite hard to implement given the diverse number of use cases there may be. Some people have all their tasks in a single file, others have all their files in a single folder, but a file per area of responsibility (work, home, personal etc.) whereas others may have a file per project inside the project folder. How would you envision such use cases be handled? That would require some notion of project akin to what Sublime Text or Atom do and that would add a whole lot more complexity to an app. This sort of thing can more easily be done with a launcher like Alfred or a tool such as Keyboard Maestro.
Just my 2 cents…
In general I think opening multiple documents in a single window “could” be an interesting direction to take TaskPaper. But it won’t happen for version 3.0, and I have very little idea of what will happen later. So yes, this is an idea I kinda like too, but I don’t see it happening anytime soon.