class MyMixin(object):

    def __init__(self, new_arg=None, *args, **kwargs):
        super(MyMixin, self).__init__(*args, **kwargs)
self.new_arg = new_arg
print(self.__class__, new_arg)

def deconstruct(self):
    name, path, args, kwargs = super(MyMixin, self).deconstruct()
kwargs['new_arg'] = self.new_arg
return name, path, args, kwargs


class MyMixinCharField(MyMixin, models.CharField):
    pass


class MyMixinModelTest(models.Model):
    myfield = MyMixinCharField(max_length=512, new_arg='myarg')
Как создать миксин поля модели django