I think we could go to a few more games if the season were to start a month or so earlier. That requires having enough warm weather teams to hold a full schedule for a few weeks until things warm up elsewhere.
Let's see where that stands.
Cold Weather (i.e. decent chance of snow in February)
New York
New Jersey
New England
Philadelphia
Montreal (but could use the domed stadium)
Toronto
Chicago
Columbus
Minnesota
Kansas City
Salt Lake City
Denver
Borderline
DC United
Seattle
Portland
Vancouver
Warm Weather
Atlanta
Orlando
Houston
Dallas
LA Galaxy
LA FC
San Jose
You need half the teams to be in warm weather zones or under a dome to play a full schedule in February. These teams would have to host a game each weekend until other cities warm up enough to start hosting. This could work well, as these cities could then host fewer games in summer months.
Right now, there are only 7 teams out of 23 that are capable of hosting a full February schedule. There are another 4 that are borderline - that could maybe host a game or two that month to ease scheduling pressure, but can't and shouldn't be expected to blow 4-5 home games under questionable weather.
Expansion could help with this. Many of the candidates are in warmer weather areas: Miami, Charlotte, Phoenix, San Diego, Tampa, and Sacramento could all host a winter schedule. Plus, any new northern team could build a stadium with a roof. Still, there is a long way to go, with 12 cold weather teams and only 7 warm weather teams.
The possibility of this is reason enough for NYCFC to include a retractable dome in their stadium plans. Aside for the other 101 reasons.