Pay! By location or By skill? I think equal work, equal pay. Pay is not just money. Profit share and stock options (or RSU).
But I think it is better to pay people for their Skills, not where they live. If you say someone live in a 50% cheaper location in terms of cost of living. But you have to live there and that’s their choice. It is about value added.
Market rate becomes higher? Adjust yearly.
People growing too fast? Promotions.