I want to use abstractmethod, but I have my own metaclasses and I don't want to build composite metaclasses using abc.ABCMeta.

Thanks to PEP 487, one approach is to facator out the abstractmethod checks from ABCMeta into a regular (non-meta) class.  So, my first suggestion is to split abc.ABC into two pieces, a parent regular class with metaclass "type":

class AbstractBaseClass:

    def __init_subclass__(cls):
        # Compute set of abstract method names
        abstracts = {name
                     for name, value in vars(cls).items()
                     if getattr(value, "__isabstractmethod__", False)}
        for base in cls.__bases__:
            for name in getattr(base, "__abstractmethods__", set()):
                value = getattr(cls, name, None)
                if getattr(value, "__isabstractmethod__", False):
        cls.__abstractmethods__ = frozenset(abstracts)

My alternative suggestion is to move this logic directly into "type" so that all classes have this logic, and then move abstractmethod into builtins.

Of course, this isn't pressing since I can do this in my own code, it's just a suggestion from a neatness standpoint.