Building on John Smith's suggestion, here is an alternative voting scheme (called the Borda count):
https://en.wikipedia.org/wiki/Borda_countThat way we avoid most situations discussed above whereby the introduction of new maps can skew the outcome of the vote. It also helps for the case where you have more than one map you like in a given week, since you vote not for one map, but rather choose an ordering of all five maps.