Eventually, Major League Soccer wants to believe that its championship game will generate national discussions of strategy and starting lineups. Not yet, though.

As Sunday’s final between the Los Angeles Galaxy and the New England Revolution approaches, most of the chatter surrounding the game is an existential discussion of what MLS is and what it will become as it exits late adolescence and enters adulthood. With one game to go in its 19th season, MLS is still trying to solve the puzzle of soccer in North America—specifically,...