I think the proposed API looks fine and should be backwards compatible since add_subparsers will currently throw an exception with a default= argument.

In case someone feels like writing a patch, you'll want to look at _SubParsersAction.__init__, which will need to grow the default= argument, and pass a different nargs= argument on. I think you'll need to define a new nargs type which means you probably also need to look at ArgumentParser._get_nargs_pattern as well.