It seems we are adding to the wishlist!  I see four so far:
1. Exposed in API, can be overridden with __array_ufunc__
2. One that converts everything to ndarray (or subclass); essentially the current implementation;
3. One that does asduckarray
4. One that assumes all arguments are arrays.

Maybe handiest would be if there is a method to coerce all relevant arguments with a function of one's choice? I.e., in the example of Stephan, one would have
if function in JUST_COERCE:
    coerced_args, coerced_kwargs = function.__coerce__(np.asanyarray, *args, **kwargs)
    return function.__implementation__(*coerced_args, **coerced_kwargs)
Actually, this might in fact work with the plan proposed here, if we allow for an extra, optional kwarg that contains the coercion function, that is
    return function.__implementation__(*args, coercion_function=np.asanyarray, **kwargs)

The possible advantage of this over yet more dunder methods is that one can fine-tune the extent to which something has to mimic an array properly (e.g., run `asanyarray` only if `shape` is not present).

It would be nice, though, if we could end up with also option 4 being available, if only because code that just can assume ndarray will be easiest to read.

All the best,