Not sure what you mean by custom form of ownership. If you are talking about fractional ownership, even that isn’t new.

Contracts around ownership are built on top of legal system. Smart contracts provide automation around defining and execution the contracts. That’s cool and can be super low cost, but apart from the automation, there isn’t much that’s different from traditional contracts. They can still be challenged in court.

For example, if you had a contract that, upon your death, transferred all your money to your nurse, your spouse could still challenge that in court if the contract was created under duress or created when you weren’t of sound mind.