Could Singleton be added?

I think that this format would be very useful to add, a lot of tournaments online do use it. It would basically just be Standard or Expanded but multi-prize Pokemon are banned. It would also help users who wish to create E- or F-tier single-prize decks to note that they aren't designed to go up against multi-prize decks. Thanks for your time!

I would like that, I have a Silver Tempest Incineroar/Silver Tempest Delphox deck that is fun to mess around with.

that sounds like gym leader challenge which is already an option 

Yes but you could add multiple copies