I might as well bring up here that a potentially overlooked benefit of these Prototype cards is that they have naturally high mana values, so if you run a lot of cards that care about that sort of thing, such as Combustible Gearhulk, they might be worth an include
They even get to show up more often in competitive play from time to time
Honestly, Arena is the reason I've gotten into Magic in such a big way
Romana II copies one of your tokens as long as it enters the battlefield on that turn
The token is exiled at the beginning of the next end step regardless of who controls it at that time, whether the exiled card is still exiled at that time, or whether Mimic Vat is still on the battlefield at that time
While the card does not see much play in meta decks, it is occasionally relevant in decks like Blissey ex to make those Pokmon incredibly difficult to knock out because of their immense HP, making Hero's Cape a phenomenal option for a few rogue decks