I know there's already a 'Game Ideas' thread and I don't want to encroach on that; I got the idea from there and, even though it's a very similar idea for a thread I still want to run with it.
Pretty much all of the best ideas (of not absolutely all) were based on an already existing franchise, usually a TV show.
Lay it out like this;
Title[of the show/movie/comic/book you'd like to see adapted into an adventure game]
I think it'd make a good Adventure Game Because... Is the nature of the [misc. medium title] be better suited to another game? Most comics or cartoons with a lot of fight scenes might not be as well suited as you might hope. Somebody suggested Pinky and the Brain on the other thread and, frankly, I think it would adapt perfectly; you'd get to build some genius device, looking for the best possible parts and tools for the job, and later use it as part of some ridiculously far-fetched scheme to take over the world. And since Pink & the Brain can never take over the world there'd always be room for future episodes.
Other things worth mentioning might licensing rights; if the [title] is old or comes from some country where it's far less likely to get a game made about it. Drawing another example from the other thread; somebody suggested Doctor Who. I can't think of a single computer game that's ever been based on a British TV franchise (with the exception of the Wallace & Gromit movies) and I really doubt there's been much interest in adapting Dr. Who, so I doubt there'd be much of a "bidding war" for the licensing rights. (I also think this would be brilliant for bringing Adventure Games to the British market. Not that we've found them too inaccessible, over here, but I think it's safe to say that most of the population don't know the genre exists).
Foreseeable Drawbacks. Probably self explanatory. If you can see any problems making your suggested show/book/whatever into a game then try to list them; if you think the franchise rights would be too expensive or possibly some other legal problem. (‘Discworld Noir’, for example, was one of the best Adventure Games I've ever played but there was some legal issue before its release that, for some reason, made copies very hard to come by). Or if there have already been a lot of games based on that particular franchise it might be hard for another one to be noticed. (Most Simpsons games prior to hit and run were ignored, though I personally loved "Bart Vs the Space mutants", or whatever it was called).