Over the years, the role-playing industry and hobby has put out many products. These products have covered and allowed campaign play in all sorts of genres from fantasy to science fiction to just plain weird. There are also plenty of "I wish" products that never get made but would seem to be ripe for a role-playing game of one sort or another. Here's some games that I would like to see.
AD&D Second Edition by Gygax
Of course, we will never get this now but Gygax did give many clues to where his design would have went. There have been numerous threads about this on various discussion boards and there is even a person/group working on what they *believe* would be a true Gygaxian 2E. In the end, nobody knows exactly what Gygax was going to do other than Gygax. This is probably one of the top hypothetical products I would want to be a reality. I suspect if this would have been made then we would still be in the third edition era of the game.
There actually was a Dune RPG released BUT it saw a very limited release after WotC bought out LUG. At that time, Wizards had Star Wars D20 and Last Unicorn Games had (iirc, of course) Star Trek and Dune. Imagine, if you will, that Wizards buys LUG and now has Star Wars, Star Trek, and Dune as RPG properties; all 3 products are powered by the same rules system also. Think about the possibilities!
There are plenty of other hypothetical products that could have been hits in the role-playing community. Possibilities that come to mind include RPGs based on The Prydain Chronicles, The Chronicles of Narnia, Image Comics, Transformers (powered by Mekton?), and Sliders just to name a few. Maybe one of these days...