there must be at least a pond,
there must be two waterfalls (wall and sea),
there must be a town plaza acre,
there must be two slopes: you can move them carefully using the acre editor
do not do weird things with the acre editor: try to keep a valid structure in the acre editor
keep at least two rocks: move them using the map editor if you need it (use right click to 'copy' the rock, place it anywhere then delete the original one)
keep enough free space for buildings in the town plaza acre: the game will freeze whenever special visitors (Redd, Gracie, Katrina...) try to put their tents there
be careful when moving any building: you can adjust building placements with the editor, but don't do weird things like placing houses in the water