I'm not really sold on this idea, and if it ends up getting implemented, it should definitely be on a per-repo basis.

I might very well turn something like this off, so the "karma" should only exist within the sphere of a repo or (forked) project.

This would just lead to a StackExchange-like gamification of the whole system.

Agreed. A dev with 100s of "karma points" on a certain PHP project should start at 0 on other projects (both of the same and different languages).