Weird things happen when shapeshifters interact with each other. I had a Shapeshifter come into play while I controlled an Unstable Shapeshifter, and I wasn't quite sure what would happen. The most sensible result seemed to be that Unstable Shapeshifter would become a 0/7 Shapeshifter, as no number for it had been chosen, so we would use a value of 0. I'm not completely convinced that this is how it works, but other outcomes (such as Unstable Shapeshifter simply dying for some reason) seemed less reasonable. Is this the correct interaction?
208.2a The card may have a characteristic-defining ability that sets its power and/or toughness according to some stated condition. (See rule 604.3.) Such an ability is worded “[This creature’s] [power or toughness] is equal to . . .” or “[This creature’s] power and toughness are each equal to . . .” This ability functions everywhere, even outside the game. If the ability needs to use a number that can’t be determined, including inside a calculation, use 0 instead of that number.
This is a specific case of the general principle in 107.2:
107.2. If anything needs to use a number that can’t be determined, either as a result or in a calculation, it uses 0 instead.
That would mean Unstable Shapeshifter is a 0/7 until you choose a number on your upkeep.
I don't think the bolded phrase can reasonably apply. The "as ... enters the battlefield" does not set Shapeshifter's P/T. It sets a choice, and a linked static ability sets the P/T. (The reason that rule is there is to handle cases where there is no static ability setting P/T.) Regardless, Shapeshifter's static ability that sets P/T is copied, and it is applied in layer 7.
This is a specific case of the general principle in 107.2:
That would mean Unstable Shapeshifter is a 0/7 until you choose a number on your upkeep.