MTG Salvation

...Walking Ballista just found a new way to kill creatures.

Turn your Saproling into Vaevictis for just 2 mana!

So correct me if I'm wrong, the wording here doesn't specify that the chosen creature needs to be in play or on the battlefield. So, does that mean you could name anything? Even if that's limited to a creature you sideboard specifically for this card, it seems like it's pretty overpowered without a specific ruling.

They addressed this in the official release notes for M19. It can only copy a creature that is on the battlefield. I think they could of worded this better for new players.

I think that probably goes without saying though. I don't think Magic would be fun to T1 island to Simian Spirit Guide to memnite to Metamorphic Alteration to progenitus would really be fun to play against.

But you could clone a Carnage Tyrant if you wanted, since choosing does not target.

Or use it as a kill spell and enchant one of your opponent's creatures and copy yours or theirs walking ballista.

Be aware the interaction with flip cards.. if you copy your opponents Nicol Bolas, the Ravager and then you pay to flip him, you'll just end up exiling your own guy! Same goes for any other flip card.

They could have worded it “target” but this way you may choose an opponents creature w/ hexproof or shroud. Or if one of your creatures has shroud you can choose it.
*NOTE* It’s similar to when you have an Aura Enchant Creature card that’s entering the BF but not from you casting it (most likely from a blink/flicker effect) And you can choose to enchant any creature on the battlefield even an opponents creature that has hexproof/shroud.

In addition, it doesn't say name. It says choose, which means (implicitly) from among available creatures on the field.

Creature cards aren't creatures until they enter the battlefield. There is a good reason why you can't target a creature in your hand with Murder to discard it.