Why not just make each touch down worth 1 point. Each successful field goal worth +3 points and each missed field goal worth - 6 points. Touch backs will be worth 0 points.
that should liven things up.
The only changes that should be made is if you block and extra point, you should be able to return it for that same point. If you pick off a pass on a two point conversion or recover a fumble, you should be able to return it for the same two.