Chips and dips of course, cheeseball and crackers, salsa, etc. Use the team colors for your serving dishes. If you want people to bring something you could have half bring an appetizer and half bring a dessert. That way you will have a variety and not all sweets. Since it is cold you could offer some hot drinks like hot cider in the crock pot.
Giant subs are always fun as well.
If you call most grocery stores ahead of time you can order a giant length sub bun and then make the sub yourself. Decorate with crepe paper, footballs, etc. Add something for both teams just in case someone comes or brings someone who is for the team that you are not rooting for. I will not share who I went to win at the risk of alienating half of the epinions readers. Just sign me, Playing it safe on the sidelines.