I think the hardest thing about this would be the HTML/Theme Editing.
I think Tumblr should open-source the theme rendering component and allow it to run independently (for making themes offline), although that's on a different page :)
Nope, just the ability to render the templates. That logic could be opened.
Then you could have a micro-tumblr setup which allows you to preview themes on your machine using fake (or real using an API, which would just dump the data required to render a page) data. Instead of having to be connected and limited to the front page preview, it would good :)
Thanks for your feedback! Team Branch