Where do we start?
Many are saying this. (Me included.)
I have a personal predilection that it really ought to be something that can work local-first. Online-optional-if-ever.
But there's so many directions to possibly go. Even if you happen to agree on that one significant narrowing.
I think there's a lot of low hanging fruit already just in pure "old school" review workflows. That's where I'm starting: first building a better diff viewer, then optimizing for code review. When I was spending 90% of my day in code editors, all the small details there mattered, now it's diffs.