This sounds very good. Only DRM-free ebooks of course, but taking the FBReader and uBook approach of supporting many formats. Offering export to MOBI format for Kindle (and others) is also a nice touch.
If you try this out, please post screenshots. For testing ePub, I typically use
Alice's Adventures in Wonderland, with screenshots
here. Note that if the CSS is being interpreted correctly, there is text formated in the shape of a mouse tail in the middle of Chapter 3. For testing MOBI I use
Dickens, Charles: A Tale of Two Cities (Illustrated), for example see
Kindle screenshots and
iLiad (FBReader) screenshots both from an earlier version of the same ebook.