i don't know if The Witcher 3 could be the last of the series but I pretty much doubt that. I know this game is based on solely on the book but still there so much events going on at the time that I really doubt that the series won't end at any time soon. So It makes me curious... are we just solely going though Garalt's history or are we ever going to one day play as a different witcher from another school? What's the chances of us going to different continents or better yet, different worlds that been mentioned in the game many times?

CD Projekt have gone on record that this is the final Witcher game.

Actually they said that this is the last Geralt game. They didn't say they won't revisit the setting.