I kind of like the 16-3 team grouping, it makes for these games being more important and wins more important instead of playing for draws. The problem with it is one of the teams playing the first game gets more time off between games than the other two teams do. That is an inherent advantage to the team that gets more off days before playing their second game.
The 12/4 team format keeps time off even for all four teams. The problem here is you have 24 advance, then you need to have 8 at of the 12 third place teams fill the brackets to get to 32 teams so you can have even numbers to get to two finalist. This to me ends up being like the NBA playoffs, you play 3 games and yet you only eliminate 16 or a third of the field after group play.
The US, Mexico and Canada will get automatic bids because they are host countries.