If every voter in every state mattered, and if it followed that at least voter turnout in non-battleground states in 2016 matched turn-out in battleground states, then the non-battleground state turnout would rise by at least 16.3%. That could be close to 20 million more votes in 2020.
But in fact if both campaigns competed to win every vote from every eligible voter in the country, they would use digital marketing, TV advertising, reinvigorated local party structures, appeals on nationally broadcast shows, newspaper ads, door knocking, and phone calls (1) to increase registration, (2) increase participation, (3) encourage people to think of joining with like-minded members of the same church, cause-related group, fellow high school graduates, family and friends -- regardless of domicile -- and voting together for the candidate of their choice. Voting would be a sport of many teams. Participation could approach registration levels, adding perhaps 50 to 70 million more voters to the process of picking the president. Everyone would have to acknowledge that the president really represented the people's choice.