I could see Apple "license" the Mac OS -- but only if the company reaches a point where they decide that the "Mac hardware" line is no longer worth retaining as an Apple-exclusive product.
They might continue to develop the Mac OS, but choose to make it available "for a price" (probably wouldn't be cheap). I'm sure a market exists for that.
Or -- they might choose to offer an "open software/hardware combo package" -- that is, an Apple designed and approved motherboard (standard PC dimensions), with a copy of the Mac OS included. The buyer would then assemble the board into an enclosure, add drives, etc. In essence, it would become what we now refer to as a Hackintosh -- only with an Apple-certified motherboard inside.
Or -- they could offer this same kind of package deal to 3rd-party manufacturers who agreed to manufacture "turnkey computers".
Again, I only see this happening if Apple loses interest in the "Mac line". Granted, it's not their largest revenue source these days, but that revenue is still substantial and it doesn't look like they'll be dropping it soon.