I have my own world totally invented for the games I run for my kids. It is filled with everything and anything that exists in the AD&D 1E books. We have loads of fun creating and exploring this world. Literally making it up as we go.
Recently I talked about making a game world based on the world of "Eragon" by Christopher Pioli.
By visiting forums like DragonsFoot, I have seen a few discussion threads where the topic of using literary worlds like Alagaësia of Eragon or Middle Earth from Tolkien's works.
In my own opinion, I think it can be a lot of fun playing in those worlds, although as a DM, there is some work to do in taking it from a novel world to an adventurable world. Especially in trying to keep within the confines of the AD&D 1E rules.
For this reason, I take the "aftermath" approach to adventuring in those worlds.
For example, instead of trying to adventure in the framework of the active storyline of those novels, I set up our time and place in the afterwards or past the conclusion of those works.
For example, using Middle Earth, We would be playing after the downfall of Sauron and all the story main characters have gone back home and moved on with their as yet un-written lives. Some people in a couple of threads on DragonsFoot forum have suggested that AD&D elves and other races would not translate well into Middle Earth because the races have different abilities and characteristics.
I say this is solved by entering Middle Earth AFTER the conclusion of the Lord of The Rings. Throughout the book(and movies) it is mentioned multiple times that the age of men is rising and the age of magical creatures such as elves is waning. Thus the elves are leaving to somewhere else.
This sets the stage to observe in our entry to Middle Earth that magic and magical characters have also changed with the times. It is thus conceivable that AD&D magic and races are now the new standard.
The same goes with my effort in the world of Eragon's Alagaësia. We step in after the end of the book "Inheritance" (the 4th and final book).
For those who haven't yet read "Inheritance" I won't include any spoilers here. Suffice it to say that I have taken some artistic liberties and brought the AD&D 1E world into post-"Inheritance" Alagaësia using the same approach as with the Middle Earth example above.
By approaching it in this manner, players can adventure through these worlds without necessarily being beholden to the forced choice factors of the narrative versions. They don't necessarily "Have" to pick sides of Galbatorix or the Varden. They can just be the character they want to be and use the geography and history of that world as the stage dressing.