This could be a worms-like rising water that increases till the map is
flooded.  Buildings that are on a field which turns to water are
destroyed including all globs inside.

Or tides:  Water just propagates a bit or withdraws -
depending on which tide is right now.

Or water withdraws constantly.  Might increase the atmosphere on
law resource maps.
Swimming pools become useless by and by.

Or leo's atlantis:
All start on the shores of the same big island that slowly vanishes.
This could be implemented by using hight-map based maps.
They'd be useful for the simple water withdraw too.

problem:
the visual transition as just switching will look ugly.

Could transient water graphics on top of the lower fields
solve this?