I like the idea. Buffalo would have a natural rivalry with Toronto and the Yanks in the AL East, joining BOS and BLT. Basically, hauling Tampa's team upward would do the trick, since they're already in the division.
Failing that, I'd move the Rockies there in a cold minute (no pun intended), rename them the Alleghanies, and let them play. Baseball really doesn't belong in Colorado - the air is too much of a permanent asset to offense and detriment to pitching. Every year you get a Vinnie Castilla sort that can't hit for snot on the road. It totally screws up the game.
The Rockie relocation would require a radical realignment. Why yes, I do happen to have a proposal, Mr. Selig:
The NL West
would drop to four teams (SF, LA, SD, ARZ).
The NL Central would become the NL South:
(ATL, HOU, StL, KC, TXS).
The NL East
would regain some old friends (NYM, PHL, Cubs, Pitt, Cincy).
I'd send the Expos/Nationals to Portland OR. They would join the AL West:
(OAK, SEA, the Angels, and POR).
The AL Central
would have MIL rejoining CLV, DET, Minny, and the Chisox.
The East, you've already seen.
Viola, 28 teams. I'd disband the Rays and Marlins and let their good players escape to happier situations.