IMO if you need a contract, you aren't communicating, and the relationship is going to fail.

With that in mind. Think about how life will intrude. What will you add into the contract when a medical situation precludes the 24/7 M/s dynamic? What will you add to the contract should the Master no longer be able to speak, or move, what about if the submissive can no longer stand in order to do the dishes, or what if the sub is unable to (insert item you have as a requirement). So on and so forth.

A contract between a Master and their slave is a nice gesture of devotion to each other, but it is baseless, and unenforceable. At best it is a tool for romance and courtship.