Nowdays there are specialty courses like "peak performance buoyancy" and "manta ray awareness specialty"....I have to confess, I just don't get it, although I am in favor of any course that keeps divers connected in some fashion.
Don't forget the DSMB specialty

Half the divers I know dive solo as do many/most Tec & CCR divers. Photographers usually end up diving solo. Divemasters dive solo with no solo training, a lot of commercial diving is done solo with surface support only.

I could talk solo all day