But the poor guy deserves retirment
True, he truly deserves it. And its was all he truly wished for. One of the things I like the most on the Witcher series, is that the protagonist is not "the designated world saviour", he was just a guy trying to make a living with normal cares and wishes, that just happened to be at the wrong places at the wrong times and got caught in terrible stuff. In Witcher 2 he just wanted to take off with Triss and have a peaceful life, and suddenly someone kills the king while he was in the room... In witcher 3, he just wanted to find his daughter...
This stuff just feels a lot more organic and immervise than the current trend of "Instant You're The Best" games like Fallout 4 or Dragon Age Inquisition, where you play for 30 minutes and you're already the designated Special One, leader of the Inquisition or General of the wasteland freedom fighters.
As for another Witcher game, Lambert is a great character , but not sure if it would be a great protagonist. There's also Ciri. But I think W3 closed the chapter for all those characters, they actually die on some of the endings, so if it comes back it will probably be a whole new set of characters.