I can virtually guarantee that this is already what happens, on some level. I think what you're getting at is that it should not randomly pick a map, but run through the list of maps in a random order, more or less. The method you described would have it remove each map from the pool of available ones as it is picked, though it could just as easily generate some random permutation of the maps, run through it like a playlist of music, and then repeat the process. Still, there isn't anything wrong with the current method. There is no evidence at all to support the claim that any map is being played more or less often than the expected 20% (based on the current 5 maps).