I think to trying meet all of these without compromises (such as privacy and performance) is basically impossible. How would one boost engagement on positive emotions or personalize without large data mining efforts, model building, and running text classification on every comment or post?
I agree they are good aspirations.
That's fair, I guess I just don't see the connection to OP. From how you phrased this I assumed you were disagreeing.
This looks cool, but I would prefer to not pass my credentials to a third party, especially for features that should probably be in the default client.
Engagement does not exclusively mean commenting or posting; voting is also engagement. If you just want to lurk, why have an account in the first place?
A few niche examples:
/r/generative /r/musicians /r/wildernessbackpacking /r/tropicalweather /r/analogcommunity /r/heavymind
Good luck convincing the non-technical users there to come here. This place is currently great for interests catered to programmers and sysadmins (speaking as one!), but artist and hobby communities are seriously lacking.