Have you ever considered introducing overseas gyms?
They could have different stat benefits just like the gyms in Torn City and give a new reason/ incentive to travel to get the benefits. It would require careful timing to land, train and then buy your goods and fly before being mugged adding a new element to travel. High risk and high gain.
I'm not going to even try to think through all the possible effects/ advantages and opportunities this might introduce, that's your job xD