Japt, 17 18 bytes

+1 byte due to fixing an error.

;@!UøX}a@ñl g1 çBö

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Chooses a random character from the alphabet and repeats it to the length of the longer input string, until it is not present in the input.

Takes input as an array of two strings.

Japt, 17 bytes

;B¬£ñl g1 çXÃkU v

Repeats the letters A-Z to the length of the longer input, removes the values in the input, and get the first item in the array.

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Old solution, 18 bytes

;@!UøX}a@ñl g1 çBö

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Chooses a random character from the alphabet and repeats it to the length of the longer input string, until it is not present in the input.

Japt, 17 bytes

;@!UøX}a@ñl o çBö

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Chooses a random character from the alphabet and repeats it to the length of the longer input string, until it is not present in the input.

Takes input as an array of two strings.

Japt, 17 18 bytes

+1 byte due to fixing an error.

;@!UøX}a@ñl g1 çBö

Try it online!

Chooses a random character from the alphabet and repeats it to the length of the longer input string, until it is not present in the input.

Takes input as an array of two strings.