I would expand to Charlotte and Portland, both in the AL. Portland is a natural rival to Seattle, Charlotte fills a gap in the AL in the South.
I would go to two divisions in each league, with 4 wild cards per league. The wild cards play a 3-game series over 4 days Mon-Thurs immediately after the end of the season, then the next round starts that Saturday. Cut out a lot of the off days in the playoffs, winning the division in the regular season and winning a playoff series in 4-6 games gets you some off days.
AL East - New York, Boston, Toronto, Baltimore, Tampa, Charlotte, Detroit, Cleveland
AL West - Chicago, Minnesota, Kansas City, Texas, Anaheim, Oakland, Portland, Seattle
NL East - New York, Philadelphia, Washington, Atlanta, Florida, Cincinnati, Pittsburgh, Milwaukee
NL West - Houston, St. Louis, Chicago, Colorado, Arizona, San Diego, Los Angeles and San Francisco.