A few quick comments

Very excited to see something native that might replace Books. A few things I noticed right off the bat, which may already be on your roadmap, when I loaded a random epub ("The Sword & Sorcery Anthology, Tachyon Press, 2012).1. The cover displays as a small thumbnail rather than a full size graphic. (Image 1)2. Switching to dark mode does not invert all text. I assume the epub specifies some text as "black," and some as generic text with no specific color. (Image 2 vs Image 1)3. The default for many epubs will be cramped and hard to read without a way to adjust line spacing. (Image 4)