I doubt the OpenZFS team would move to GPLv2 if they were able to relicense to anything they wanted. Given their close association with FreeBSD, BSD-2 or a similar permissive license wouldn't shock me.
But it's an academic exercise anyway, since it seems Oracle has no intention of allowing them to relicense.
So the OpenZFS team is not exactly interested in moving to GPLv2, because it would break multiple platforms.