I think the universe itself is amazing and the life of a Witcher lends itself exceptionally well to an RPG, open-world or otherwise.
There's a lot of possible stories left to tell in this world because the characters from the books have created a really high bar to aim for future installments. The question really comes down to when a sequel should take place. The endings in Witcher 3 have some pretty big implications that are quite different from each other, so if you're going to set a new game in the near future, you'd likely have to set it in another part of the world and reference what happened as a result of Witcher 3's actions, or have some major event create so much havoc in the world that it will change things enough to create a common starting point.
Or you could go into the past or the future, then all bets are off.
As for who you should play? I would love to play a brand new Witcher. Someone who goes through the Trial of Grasses and sets off on the path. You then learn to apply your theoretical knowledge in the cutthroat world, meet people, make friends, make enemies and maybe meet people that could become love interests. Sequels to this first game would show your Witcher's progressive growth and mastery of his trade. You start as a greenhorn and become more and more hardened by the world and how people treat you. Eventually you should be as knowledgeable and capable as Geralt.
Seriously, I just loved moments in Witcher 3 where Geralt showed off his incredible knowledge about monsters to clients. You should be able to see your new Witcher's progression to that point.