The only way these cards can be used is as proxies in sleeves, which would happen in casual play
Just remember that whichever mode you choose to cast becomes the mana value of the card on the stack and on the battlefield, unlike other, similar built-in cost reduction mechanics like Suspend or Miracle, s o a commander like Yuriko, the Tigers Shadow very much likes Prototype cards, while Vial Smasher the Fierce does not
On the other hand, for small kitchen-table playgroups where the participants know each other and can agree on variations beforehand, relaxing and tweaking (or, alternately, locking down) the Commander rule can lead to some interesting variations, almost like alternate histories
The Legendary Treasures RC card is a fan favorite, and the Crown Zenith one is a great modern pick
Once you maintain the crown, you can establish annoying loops with cards like Cathar Commando and Selfless Spirit that sacrifice themselves to accrue value