@dansup there used to be a full-blown (non-federated) remake of Stackoverflow in a GH project. Firing it up would give nearly the same UI and a whole bunch of the gamification features.
I was only mildly interested at the time, and did not star or anything. Later on, seeing "federated SO" thread for the N-th time I tried to find it again on multiple occasions. No luck, unfortunately.
Such project would be a great basis to add #ActivityPub to.