There is currently no way to automatically replace all cliff by beaches, or set the default surrounding terrain to beach instead of cliff. Could it be possible to add this feature?

Either allow changing the default terrain, either allow us to setup a % ratio between cliff vs beach in the XML to change the results of the generator (with of course the possibility to set 100% beach and 0% cliff for no cliff). Or have a special function in the map editor that replaces all cliff by beaches. But the problem is that it must be done manually.