Here's a sketch for a possible decorator factory:

def parameters(params, *, builder=_default_param_builder):
    def make_parameterized_test(f):
        return ParameterizedTest(f, params, builder)
    return make_parameterized_test

(default builder would be the one I sketched earlier)

Example usage:

ExampleParams = namedtuple('ExampleParams', 'x y')
all_cases = [ExampleParams(x, y) for x in range(3) for y in range(3)]

def x_y_naming(name, cases):
    for idx, case in enumerate(cases, start=1):
        x, y = case
        yield ("{}_{}_{}_{}".format(name, idx, x, y), case)

class ExampleTestCase(TestCase):
  @parameters(all_cases)
  def test_example_auto_naming(self, x, y):
      self.assertNotEqual(x, y) # fails sometimes :)

  @parameters(all_cases, builder=x_y_naming)
  def test_example_custom_naming(self, x, y):
      self.assertNotEqual(x, y) # fails sometimes :)

Once defined, you're far better equipped than I am to decide how TestLoader and TestCase can best cooperate to generate appropriate test operations and error messages.