It'd be nice for individual zlevels to have their own max_x/max_y values for various purposes (such as making zlevels at runtime for the interiors of various objects. eg: stuff a person into a bag of holding, and they get their own room, without the need of dedicating an entire, full-sized zlevel). being able to delete these would also be nice.

In terms of syntax:

world.zlevels list, containing zlevel datums.

each of these zlevel datums would contain:
the actual z value
max_x
max_y

ideally the order of the zlevels list would reflect the zlevel's z value. AE: world.zlevels[1] would give you the world's first zlevel.

You could go as crazy as you'd like for the syntax for creating it. presumably adding to world.maxz would add a new datum; but the potential for having more is up there, such as a `new` argument to load a dmm.