Golden cash- a weapon factory owner that put his eyes on equestrian magic as the new kill tactics and want to use the rainbooms powers for his weapons

The dark lights- Cover band of the rainbooms that tries to steal their ideas and fame( kind of the mean 7 if you may)

Down luck- a magic hater fanatic that want to destroy equestria before they will destroy his world.

Black sun- A mixture between the darkness and an obsessed fan of Sunset

Interesting ideas.