What would be a good second dog for my dog I already have?

I don't plan on getting a second dog for a while if ever but I thought it'd be fun to see what you would suggest.

Some info about my dog:

LOVES to play

Rough when plays but will back off if the dog tells it to

Is intelligent

Gets along with both male and females

Can bring out the fun side to an otherwise shy/unsure dog

Boxer/pit mix so a dog that can keep up with how rough he plays is a good idea

53 pounds as of now, and is around the length of a border collie

Can be a bit dominating at first, really is just testing to see where he stands with another dog. He doesn't dominate again if he knows the dog doesn't like it.

My requirements:

Grooming isn't a big deal for me, I'm willing to learn how to groom a long-haired dog.

Isn't dominating in nature, one some-what dominate dog is enough for me to handle right now. :P

Isn't a guard dog, I really don't need a guard dog.

Able to keep up with my other dog, but I'd prefer a medium energy dog.

Is about medium size wise.

Does okay in condo, no backyard.

Active in the house.

If you have any other questions feel free to ask.

Also: You don't have to say adopt a dog or a mutt, I'm curious about purebreds that would get along with mine. I would probably end up adopting anyway this is just for fun.