Fill up to duplicate ranges
Perl 6, 37 33 bytes

-4 bytes thanks to nwellnhof!



{^.keys.max+1 xx.values.max∖$_}


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Anonymous code block that takes a Bag and returns a Bag of values.

Explanation:

{                             } # Anonymous code block
 ^.keys.max+1  # Create a range from 1 to the maximum value of the list
              xx  # Multiply the list by:
                .values.max      # The amount of the most common element
                           ∖$_   # Subtract the original Bag