I have suggested it before:
Why not move the Masternode network completely and solely into the IPv6 space.
Every node could have an arbitrary amount of addresses, bound to an arbitrary amount of wallets.
The node addresses could even be changed all the time by some maintenance protocol.
How would non-IPv6 clients be able to connect to a masternode? I believe most end users like myself currently do not have access to IPv6 through their ISP.