A neat feature would be to have a not-out-in-front-of-the-entire-world area where it was possible to change things around on a theme, fuss with the coding, and so on. Then, once it was right, make it visible like a regular theme. This would also have the advantage of making it easy to backtrack to a working version after messing things up ;-}
I know I could set up a local server on my computer and supposedly do it that way, but in fact there are differences between the trial versions and the actual "real" server versions.
I don't know enough about programming to know how difficult making such a "sandbox" would be. Conceptually, it seems that it should work to set up some sort of /tmp space for everything until changes are finalized. But I do know enough programming to know that nothing is ever simple if it seems like it should be. So this is just a thought / suggestion.