Amazon US and Amazon UK are already here, although some products might be missing due to complexity of integration and API restrictions.
Newegg is a great idea and we'll add this intergration to the list of priority tasks.
Itch.io is a difficult one - their database is really big, but majority of their products are not featured on GG.deals. Creating a product page for each game from Itch.io might be challenging. Nevertheless, we'll look into it.