The founding fathers took care of that, they gaveus the Ammendment process.
Cruel and unusal is really easy for me, just check the statues of what was allowed by the 13 states and check for any over rulings in the fist 50-100 years. OK, maybe that's a nod to stare decisis, but I'm lazy and stare decisis is for us lazy jurists.
Is cruel and usnusal what is deemed such in each generation, or fixed on stone by the morays of a snapshot in time? What did the founders intend? Nobody knows, frankly, and those who suggest otherwise, are dissembling.