By adding a different mine for each rank and with better ores for higher ranks would fix the overpopulation of the VIP mine and also give donators more of the advantage they paid for. Though I think restricting each rank to its designated mine (MVP can't enter VIP mine) should be implemented with this to again help with the overpopulation of a single mine. Another solution could be to make 4 mines like the VIP mine and allow access to them depending on your rank (VIP gets access to one mine, MVP+ gets access to all four).