Perhaps we could do something like what many projects do (e.g. OpenBSD, Linux, GNU, many many more) and have mailing lists for each project where people can send patches if they want them to be reviewed?

Actually I guess that is overkill, since we have a forum we could do that in. But I greatly support the practice of sending patches somewhere (forum, mailing list, wherever) and discussing them in public, where there will be a record of: what the patches were, how the review process changed the patches, what the discussion was etc. I think this is important for future contributors, so they can learn and make their contributions better.

Your example of Alice sending Bob patches is so simple you didn't even explain it, since it's trivially obvious what happens (they mail back and forth, Bob eventually is satisfied and applies the patches).

Recall that the Linux kernel is developed this way, so the tooling around emailing patches is excellent: look at git-send-email and git-am, for example. It sends the commit and applies it, preserving author, time data and GPG signatures too.

(Note: I am not specifically advocating for mailed patches, but just putting patches somewhere and talking about them as a group before applying them, at least for big patches adding lots of new functionality)