First - I am neither for or against, but for discussions sake...

If they do this, there is no way IMO that it could start in 2020. Next season at minimum would have to be the target date.
So, 32 NFL teams makes 16 extra games that need to be played somewhere that is relatively neutral.
My first thought was that cities that recently had NFL teams leave; St. Louis, San Diego and Oakland could host a game.
Non traditional but regional rivalries should be the focus, for example the Bears and the Colts. I don't want three games with Green Bay.
How about those two play in a neutral city like South Bend at Notre Dame's stadium which I think can hold about 80K.
Not sure about the timing, but again IMO this game could be played in week 1, possibly in conjunction with Labor Day weekend.
Other options would be Cleveland, Pittsburgh or even Tennessee.

College programs that I think have big enough stadiums and regional, non - conference teams; Alabama (Bucs - Jaguars), Texas/Baylor (Cowboys - Texans), LSU (Saints - Titans), Tennessee (Titans - Panthers)