Personally, I would say the Witcher 3 is the perhaps the greatest game of all time, I don't think there are any major changes I would make other than to improve the water texture, make the main characters appear in the world after the game has ended (with generic dialogue), and never have any members of the wild hunt remove their helmets. Beyond that, the Witcher 3 is the closest thing a game has ever come to perfect.
No game, let alone a 3rd person action RPG, has made me experience such a wide range of emotions, and left me such fond memories. From visiting Crookback bog for the first time and following the trail of body parts made to look like candy, to being shipwrecked off the coast of Skellige and realising that there is another world map nearly 60 hours into the game. Even little things like following the story of Ciri while she was on Skellige, specifically her encounter with Skjall, and the mission where you have brought all of your closest allies together in order to defend Kaer Morhen (which is probably my favorite mission of any game, ever).
I got the ending in which Ciri becomes empress (which while it is technically the best ending, because she will likely live to a good age in comfort while making a difference to the world, feels like a very sad ending because while she knows she has to become empress, she doesn't want to leave Geralt and her 'family'), and I left with Triss, rather than Yennifer.