If the question is open-ended like your request seems to be, then here are some suggestions.
1. You can compare and contrast high school football to college football to professional football. What are the differences and what are the similarities? More interestingly, what causes these differences?
2. You could compare football to another sport, such as soccer or ruby. You can even have an international emphasis.
3. Or you can study the history of football. In this way you can compare football of the past when it was an ivy league sport among elite colleges to what it has become.
I think that the topic needs some more scope and definition. I think that if the paper is something in the realm of history of football, examining how the modern NFL was borne out of the original franchises and then its merger with the AFL would be worthwhile. If the paper is examining how the game of football has cultural meaning, I would propose examining the power of the Super Bowl as something that has helped to establish cultural relevance over the last forty or so years. If the paper is about economics, I would propose talking about the history of labor agreements in the NFL, and discussing the prospect of a work stoppage in the next two seasons. Along these lines of modern challenges in the NFL, addressing how the league is dealing with head injuries and concussions as well as how former players, now retired, are being treated is quite a relevant topic. The scope of your paper needs to be defined in a more narrow manner and then more wortwhile discourse can emerge.
I think that you need to narrow your topic a bit. Football is a very broad topic, and there are many different directions you can go in in terms of comparing and contrasting the sport. Here are some different suggestions:
1) You could compare and contrast college football to professional football. Even from here there are a variety of topics you can choose. For example, how the rules are different, the playing/coaching style, how players are different, even the fans and stadiums
2) You could compare and contrast football in the early 20th century to football in the early 21st century
3)Compare and contrast different teams in professional or college football. How are they coached different, how to they practice, etc.
Hope this helps!