As I have read Steven Erikson's series of books titled Malazan Book of the Fallen, I came upon the realisation that I would love to play as a character in that universe. Another great realisation was that making a game after a book has turned a certain franchise into a smashing success, while it turned some into a gigantic flop. The latter I attribute mainly to the half-assery of the studio that was making the game. The smashing success are the Witcher games, while a gigantic flop is the Game of Thrones game. Both are role playing games, yet one really understands its universe, while the other is crippled on every twist and turn by the design decisions the developers made.
This alone should serve as a precautionary tale that a game based on a fantasy series can go both good or terribly wrong. As far as I know, CD Projekt RED took the Witcher world and built upon it using their own imagination while still staying loyal to the source material. While the developers of Game of Thrones game chose the easy path and payed dearly for it. Voices of the TV series actors, insignias from a TV series, locations like they were in TV series. When you are developing a game, you are given a gift that TV series cannot have. You can stay true to the source material. You do not have to use tricks or workarounds in order to make a castle. In a game, Castle Black can look like the Castle Black George R.R. Martin envisioned, not a provincial fortress TV series had to settle down with.