there are certain rules of the artistic kind, but it depends upon the subject and the Idea of you dio. by example, its not very good to place anything parallel to the edge of the base, as it looks like you made your subject to fit your base, instead of the opposite.
you should also go for some sort of balance, if there is some big event on one side then there should be another either just as big, or many smaller.
also, don't forget to remove that dead space, if there is nothing really there then consider an smaller base. aaannnddd, finally, every event that happens in your dio has to be in some way directed to the subject to support the main idea and avoid confusion to the viewer. make sure that you understand the message of your diorama very clearly so that the occasional viewer will too. bye