Your best bet may be to develop a processor-agnostic POS system, meaning that your system will work with most major processors. Otherwise you will be selling two solutions instead of one. The first solution will be the POS system itself, but the most difficult sale will be getting an established company to switch credit card processors to utilize your system. Unless you can offer them some financial incentive to switch you will be the underdog from the starting gate.
Also, since every business is different, having a one-size-fits-all credit card processor is impossible. Different volume levels, monthly charge volume, types of customers, types of business, and other risk factors will all directly influence the rates a processor will give your customer.