tl;dr: my wish is a code review convention for git-notes and a friendly UI on top.

If only Git would allow storing reviews and issues in the repo proper. Something like Git notes with conventions to refer to files/lines in a file/different revisions etc. Something similar to Fossil.

This would solve half of the problem. The other half would be the UI layer.

Now the reviews live in a database, outside of the repo, and can't be downloaded locally for offline work.

This would allow doing code reviews and managing issues e.g. while commuting in a train. And everything would be pretty fast compared to waiting for a web page to load for every file you want to review and connecting to a server for every comment you write.