I don't add any spices --- just a few slices of Kraft fat-free cheese singles and some salsa or ketchup. Sometimes I add a slice of tomato.
You could try adding pepper. I tried a pepper burger at a McDonalds in Taiwan once and it was really good.
I just checked at the store and the bison patties I buy are made by "Minnesota Premium Bison". They seem to have the leanest patties. The other brands don't seem to be as lean --- it is visibly noticable.