Here's an idea for a plugin:
When comparing editions of ebooks, I frequently want to do a diff between the two books to see what the differences in the actual text are. Sometimes you'll come across one edition with nice formatting, but another edition with better proofreading. Other times I'm trying to look to see if there are differences between ebooks claiming to be from significantly different print editions.
An example is the Arthur Conan Doyle books discussed in this thread:
The problem here is you can't just use a diff tool on the raw html, you'll never get anywhere that way.
The solution that seems to work the best is to convert both books to text (enable 'smarten punctuation' to normalize punctuation differences). Then you can load the two editions of the book in a visual diff tool and get a very decent idea of the differences between the two editions.
There's a lot of manual steps to be done here, and it seems like an excellent workflow for a plugin. Basically highlight the two books you want to compare, and the plugin runs those two files through the conversion pipeline, outputs to temp files, and calls up the diff tool of your choice.
The most complex bit is figuring out how to pass multiple arguments to the diff tools, I imagine this varies from platform to platform and I'm not sure if this is something that can easily be done/configured by the user. Most of them seem to support this by using terminal/shell commands - can a plugin launch an external app this way?