Add the ability to display games which you are missing in order to have a complete set

You can do this already.  If you visit a console page like N64

https://www.pricecharting.com/console/nintendo-64

You will see a blue "+collection" link on the side of each item.  If you have added it, that button changes to orange color.  You can quickly look down the list and see all that are blue and are missing vs orange that are already in your collection.

Same thing works with search results too.

I hope that makes sense.