I'd say a comic/graphic novel has become an interesting medium for world building. A much more daunting task because you are really restrained by art and storytelling. My friend during most of his college career worked on a graphic novel of sorts. It never really went anywhere with his access to the supplies to produce a graphic novel were really limited, but it was an interesting goal. I myself have always seen myself writing a novel. A world I've been working on and off on for two years is definitely a novel in my head. The other which has had significantly less of my focus is really unknown in design. I will probably be sharing the second one soon because I think its a pretty interesting concept on what is out there right now, it's just early in its development.
Can't believe I forgot about graphic novels! Thanks, I've added that to the list. Graphic novels are possible even without physical supplies, since you can create quality digital art with free software, if you have the skill. For your second idea, if you're unsure what route to take for it, lay it out, and others can certainly offer ideas on how they'd like to see it made. I've found that some of my worlds search for a while for a medium to present them in before finding a home. I've got one world, for instance, which I considered making a series of short stories, but has changed to become a tabletop RPG. You never know where it might go!