[chessplayer]

I suppose we could set up a playoff between the NFL champion and the Canadian Football League champion. Common rules for the game could be set because the NFL and CFL have some different rules. Canada is the only other country that plays football the way the US does.

[safeasthebanks]

True, although for me (and I realize this is somewhat just my own preference), world championships need to be played for by countries (national teams) rather than two (or more) league teams.

So to tweek your example, the game would be between teams made up of the best Canadian gridiron players (whether they play in NFL or CFL) vs. the best American players (again, either from NFl or CFL, though clearly at this point they would all be from the NFL).