Some discussion today led me to believe that a post segwit hard fork could
include:

1MB old tx non-witness segment
XMB new segwit non-witness segment
XMB witness segment

By partitioning off old transactions, it allows users of older, more
expensive validation transactions to continue using them, albeit with
higher fees required for the restricted space.

New segwit blocks, which don't have the hashing problem could be included
in the new non-witness segment of the block.