I feel like Twitter would be way better if people had different topics you could subscribe (or not) to.
You're really better off with traditional thread-based forums to have conversation topics. Muting somebody's entire existence just because they occasionally dirty the general feed with 'offtopic' just doesn't seem like a sustainable way to use social media.
It’s nothing personal for me not to follow someone, so I don’t really see it as muting their whole existence.
All services would be better if they were more like Google+ (but didn't share name with a despised effort to crush pseudonyms and wasn't owned by a company that buried it as soon as they had been forced to iron out the wrinkles ;-)
Opt in would be nice, I am sure high follower people would rather go to the effort of categorising their tweets rather than losing followers when they decide to Tweet about their local sports.