I don't see the point of this discussion about default font & font size. What is nice with eBook is that you can change these fonts and sizes according to your taste and your eyes. The bigger the screen, the more words you can display, or the bigger fonts you can reasonably use ... and the more comfortable you'll be. And so, eBook reading is a lot more comfortable on our NIT than on iPhone.
Sorry, not gonna work. There are *always* many, sometimes competing applications for the same purpose. It might be not what you're looking for, but "In varietate concordia" is one of the defining characteristics of OSS. The One Killer Application To Rule Them All is certainly a working business model, but not for platforms build around OSS.