There are a number of fantasy or science fiction settings that I think would be fun to explore. This page is sort of note keeping section. I'll probably never do much with these settings, but would probably be willing to buy a d20 or GURPS version if it was ever written. There are even more books that have a plot device I see potential in for gaming.
Most of the settings I find interesting for the background ideas and world design. The plot of the actual books is usually not very useful to me. I don't want to create a game world where the players are trying to follow the plot of the books. I don't like to set up the players as having to save the world
The oldest setting I have wanted to see converted into a gaming world is Earthsea setting, created by Ursula K. LeGuin. As far as I know there has been no licensed version in any gaming system.
Another setting that I think has potential is Jack Chalker's Soul Rider series. It's a world where you don't have to worry to much about simulation of real planets. Most of the world is called Flux, and anything goes since it's basically controlled by magic. The normal stuff, called Anchor, is small islands of reality in Flux. I'm thinking BESM would be a good fit for this world. I read the first book in the early 1990's, but never got to finish the series.
A more recently published series is Harry Turtledove's Darkness series. It is a world where magic is very common and is used instead of technology. The look and feel of the world is the tech level of 1930's and 1940's. The plot line is essentially World War 2, with each of the major powers in our world having a sort of analog in the fantasy world. What I like is how magic is used instead of technology, ranging from magical ray rifles to refrigerators. I especially like that magic is not very flambouyant, unlike what I have seen of Wizard's of the Coasts' Eberron setting. The other nice thing is that the magic seems to be more integrated into society. Magic isn't this special effects and combat setup like d20 usually is. World of Darkness magic has an effect on society. For example, travel is common via the ley line trains, and thus people take vacations or visit relatives in distant cities. The trains aren't just for adventuerers.
Plot devices are ideas or concepts, often of a technological, unknown physical or magical nature, that can be the basis for a book or game. A good plot device is re-usable outside of it's originial story or series. Also see my list of SF&F books by genre for other books using some of these plot devices.
Probably the most widely know example of a cross dimensional gate in use is the TV show Sliders. A cross dimensional gate permits people to travel from our world to an alternate time line. It is the same year and same location, but the dimension traveled to has a different history and thus a different now. Other examples would include S.M. Stirling's Conquistador, or Charles Stross's The Family Trade. The device could be a portable timer (Sliders), permanent gate (Conquistador) or special genetic ability (The Family Trade). What the characters do depends partially upon the type of device. In Sliders, the party has little control over how often they change worlds or what world they go to. This makes for good random episodes and adventures. In Conquistador, the gate is a fluke of technology and goes only between two specific worlds; in The Family Trade the ability is genetic to travel between two worlds. This makes exploration & exploitation and long term planning the goal. The cross dimensional gate is also a good way to explain why characters in our world have special powers such as magic.
Time travel has been a plot element since HG Wells wrote his famous work The Time Machine. What I find more interesting than just time travel are the rules that various authors create about the time travel device in their stories.