Err, I understand your point about footprints, but you make some massive lapses in that.

Portland fans will not support Seattle. There is a strong rivalry between the two cities that won't be bridged. They only support the M's and the Seahawks because Portland generally doesn't have that many MLB or NFL fans.

Soccer is a completely different animal for Portland. They have one of the longest soccer traditions in the states, along with Vancouver and Seattle. The Timbers are one of the best supported USL teams, and along with that came hatred of the Sounders and Whitecaps. Imagine if the MLS had have ignored New England in the hopes that Boston fans would support the New York team. By your model, FC Dallas and Houston shouldn't exist. Philly should be the last place on earth to get a team since they show up to the NE and NY games anyways to jeer them right?

The footprint model is a flawed idea. Television soccer fans are already ignoring the MLS in favor of the premier league etc. The real goal should be to draw steady crowds and make the atmosphere interesting. Find me groups of Miami fans that are going to travel to NY or TX in the bulk that Portland fans will travel to Seattle and Vancouver. I guess basically I'm saying, don't try and ram the MLS down the throat of some area when there are cities with existing rivals interested in getting in the fray.

The more interesting the games actually get at the stadium, then maybe a footprint model to increase tv viewers. Shouldn't focus on the fans at home right now though, when there is plenty of space at most stadiums.