One of the features I have planned for much further down the road is something I call "shadow blogs", something that stems directly from my frustrations with blogging. The idea is that you can create a virtual clone of a blog that you use as a staging area for changes to a theme, so that such changes can be tried out in an interactive way and then applied all at once.
Here's how this would work:
One of the beauties of this approach is that you wouldn't have to actually copy anything to use live data on the new theme -- e.g., you don't have to make a clone of your blog, then manually sync changes to every post between them. The original blog remains live and accessible, and the new one simply uses the same data, save for its presentation.
The downside is that you'd need space to rebuilt a copy of the existing blog with the new theme, which I don't like. My plan is to eventually have it so that you'd be able to interactively explore the shadow blog with each page being built on the fly, as needed. Alternatively, you could build only the pages you needed to preview.
Either way, I think this would be a huge boon to those who want to revamp their blogs from the ground up, but find using live data for that process to be a terrible hassle.