Aren't BCs supposed to be high-energy, bright, herding dogs? I would try to match with a similar dog, emphasis on the high-energy & sharp wits. I think the herding instinct is less important to match. Size matters less, but I would avoid hte extremes (Chihuaua/Great Danes).
Matter of fact, if you matched her with, say, a retriever, you could have some interesting interactions. My sister & B-in-Law have two pound pups: a 3/4 lab mutt and a mostly border collie mutt (seasoned with german shepherd, maybe). They invented "The Game," which consists of a human throwing a ball a ways, the 3/4 lab attempting to retrieve the ball & the mostly BC trying to herd the 3/4 lab into a corner, dogs barking up a storm and mock-fighting. They eventually make it back to the human, saliva-covered and panting in great gasps...ready for another round of "The Game." They'll do it until they fall out from exhaustion and head to the water bowl.
