For omitting some defaults, @serhiy.storchaka already added support to initialize some ast nodes with some default values (optional fields). An example;
>>> ast.Constant().kind is None
True

This isn't exactly a default value, but some kind of class attribute. I think we can push this one step further and initialize all kinds of default values (both optionals and sequences). An example;
>>> func = ast.FunctionDef("easy_func", ast.arguments(), body=[ast.Pass()])
>>> func = ast.fix_missing_locations(func)
>>> exec(compile(ast.Module(body=[func]), "<stdin>", "exec"))
>>> easy_func()

compared to this (other way around, compiler gives errors so does most of the ast based tool, including ast.unparser)
>>> func = ast.FunctionDef("easy_func", ast.arguments(posonlyargs=[], args=[], kwonlyargs=[], kw_defaults=[], defaults=[]), decorator_list=[], body=[ast.Pass()])
>>> func = ast.fix_missing_locations(func)
>>> exec(compile(ast.Module(body=[func], type_ignores=[]), "<stdin>", "exec"))
>>> easy_func()