Pretty much that is it! I can play with the fees but there are upgrades that can be done like pushing it to twitter so you know when someone throws down a alot of coin and you want to be the first to take the offer.
If other reputable sportsbooks take large action at 10% fee (which most are a lot less actually) why would anyone post potential bets on your site when there's no guarantee the bets will get matched. A site called btcsportsmatch.com does exactly what you offer almost and has been in business 2+ years and 1% fee.