I'm a Kim Ng fan myself; but I think the "experienced GM" parameter will exclude her for now.
My hope is that Mather, as he gathers data, will realize that the LAD/CHC/AZ/Cleveland/Toronto system of a business CEO and a strong "baseball guy" COO above the GM is better than putting the GM out there with little back-up. His comments about the job indicate he may be starting to lean that way. At the very least he understands the need for strong, smart AGMs - and probably more than one.
A Beinfest-type backup to an analytical GM, with another strong individual for player development is my preferred outcome. Beinfest-DiPoto-Ng, with a planned transition if things go right would be very nice. I also think Jim Beattie is wasted as a scout with his development experience and Mariner background.
I think the BIG failing in Z's tenure was in development. My suspicion is that there have been too many "advisors" to the GM and not enough line accountability. That has to be job #1 for any new GM or "Baseball-Management Team" coming in. Strong, accountable leadership can fix a lot of things. Look at the Cubs!