I see the power of Kindle in the ease of buying books. I see the device as a storefront to Amazon's e-book collection.
To be more massively successful, such a device should come with access to a sync service (or several), and access to e-book stores. IDK if Amazon has an API for buying books, but definitely some e-book sellers with APIs exist.
And yes, of course it should allow uploading DRM-free books of your choice, and maybe integrate nicely with Calibre.
The software part of the project looks much bugger and gnarlier than the hardware part.