3 East Coast Clubs, 2 Clubs from the middle of the country and 1 from the West Coast. I'd say another West Coast Team (I'm partial to the Giants especially as they are the Dodgers natural rivals from their days in New York.)
The other thing I notice is you then have natural rivalries Bos-NYY, Cubs-Cardinals and if you go SF, Dodgers-Giants. The odd team out here is the Phils so I'd look at someone NL Easty. The Mets and Phils are a nice one, or you could have a Pennsylvania rivalry with the Pirates...hell the Braves (original National League franchise in Boston and my dad's favorite growing up) and Nationals-Senators also would be good (Philly-DC was on my TV just a little while ago and with Bryce in town the Nats fans were incensed.) One wild card here, Les Expos. Montreal and the Phillies were both NL East teams and it puts a team in Canada. Last one as I think about it the Reds would be a good team they were the first NL Team outside of Northeast if you go far enough back.
I guess my votes are the Giants and the Reds (or the Expos...)
