Okay, here's a list of the US & Canada metro areas greater than 1 million in population. "X" marks an MLS city; "XX" is obviously 2 clubs. "Y" are candidate cities. "~" is for metro areas that are at least arguably covered by a club. (Providence is the canonical example of this, as Gillette is actually closer to Providence than Boston.)

Rank MLS Metro Area 1 XX New York-Northern New Jersey-Long Island 2 XX Los Angeles-Long Beach-Santa Ana 3 X Chicago-Joliet-Naperville 4 X Dallas-Fort Worth-Arlington 5 X Philadelphia-Camden-Wilmington 6 X Houston-Sugar Land-Baytown 7 X Toronto 8 X Washington-Arlington-Alexandria 9 Y Miami-Fort Lauderdale-Pompano Beach 10 X Atlanta-Sandy Springs-Marietta 11 X Boston-Cambridge-Quincy 12 ~ San Francisco-Oakland-Fremont 13 Y Detroit-Warren-Livonia 14 ~ Riverside-San Bernardino-Ontario 15 Y Phoenix-Mesa-Glendale 16 X Montreal 17 X Seattle-Tacoma-Bellevue 18 X Minneapolis–St. Paul–Bloomington 19 San Diego-Carlsbad-San Marcos 20 St. Louis 21 Tampa-St. Petersburg-Clearwater 22 Baltimore-Towson 23 X Denver-Aurora-Broomfield 24 X Vancouver 25 Pittsburgh 26 X Portland-Vancouver-Hillsboro 27 Y Sacramento–Arden-Arcade–Roseville 28 Y San Antonio-New Braunfels 29 X Orlando-Kissimmee-Sanford 30 Y Cincinnati-Middletown 31 Cleveland-Elyria-Mentor 32 X Kansas City 33 Las Vegas-Paradise 34 X San Jose-Sunnyvale-Santa Clara 35 X Columbus 36 Y Charlotte-Gastonia-Rock Hill 37 Y Indianapolis-Carmel 38 Austin-Round Rock-San Marcos 39 Virginia Beach-Norfolk-Newport News 40 ~ Providence-New Bedford-Fall River 41 Y Nashville-Davidson–Murfreesboro–Franklin 42 Milwaukee-Waukesha-West Allis 43 Jacksonville 44 Memphis 45 Louisville/Jefferson County 46 Richmond 47 Oklahoma City 48 Ottawa-Gatineau 49 Calgary 50 Hartford-West Hartford-East Hartford 51 New Orleans-Metairie-Kenner 52 Edmonton 53 Buffalo-Niagara Falls 54 Raleigh-Cary 55 Birmingham-Hoover 56 X Salt Lake City 57 Rochester