Is there an adjective in English that describes something that didn't exist before but now it does and it makes you happy?

adjectives single-word-requests

Solution 1:

This is a bit mundane, but fits your description:

gap-filling

Batman #687 depicts a number of gap-filling moments which longtime readers probably expected to see, but which weren't necessary to the stories other writers wanted to tell.

https://www.cbr.com/shades-of-grayson-rounding-up-junes-bat-books/

Also seems to be used commonly in law 1 and government, such as:

the Government's gap-filling regulation

--Supreme Court's Decision in Home Concrete

and

gap-filling medical equipment

-- US government agency document

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