In 5e it would have to be a beast. A very magical sword if you ask me.
When the official latest D&D movie doesn’t care about the rules (druid wildshapes into an Owlbear) why should the players?
rule of cool to me means you bend the rules to make the players feel badass, it usually doesn’t mean you disregard the rules completely and do whatever you want. At that point just run a systemless narrative storytelling game.
As for polymorph turning someone into an object, there is a spell that does exactly that: true polymorph.
I am by no means a rules absolutist, some of the best moments I’ve had in games were certainly not RAW, but from experience it feels really shitty to allow individual players to do things that their abilities specifically don’t allow, because often that overshadows other players that either specialized into some abilities that are now obsolete, or might’ve had creative alternative approaches to the problem
Which I always find kind of hilarious since it’s basically expressed on the first page of the DMG.
Only if it’s the 4th level version, which is impermanent anyway. 9th level polymorph has rules for objects: https://roll20.net/compendium/dnd5e/True Polymorph#content