In the future, McRegion and PMAnvil will become deprecated legacy formats which we won't support for writing anymore. However, this brings complications because Anvil (which would not be deprecated) would be sandwiched between its deprecated base (McRegion) and its deprecated child (PMAnvil), complicating the implementation (particularly for PMAnvil).
This abstracts away the generic Region-based world functionality into a separate class, making McRegion, Anvil and PMAnvil all inherit from it directly. Since only the chunk formats are different, they are more accurately siblings rather than parent-child (although Anvil and PMAnvil are cousins, ish).
