I am sort of working on the same thing. It is mostly for my own use so far, I might share it at some point.

What I’ve focused on besides the basic features of the arc sidebar is memory usage.

I am a tab power user, in Brave I had 370 tabs “open”. Each tab still takes resources when asleep though. But I use these tabs more as bookmarks. So they are asleep 99.99% of the time, if not more.

The browser I’ve been working on looks a lot like Crest because they both take Arc as a starting point. I am interested to see where they both move towards from there.