I have got the world-building bug something awful. With the addition of Al-Qadim to my gaming collection, and the fact that I am currently teaching Beowulf to two of my classes, I find myself wanting to sketch out new fantasy worlds.
Fortunately for me, most of my current campaign's world has been left undeveloped or vaguely sketched out; the only environs that got much detail were the town of Ersel and the areas of note immediately surrounding it. (These being mostly dungeons: the old cemetery, a nearby crumbled fortress, Radamant's tower, etc.)
Another thing I have been thinking about is racial magic. That's a can of worms I'm not sure I want to open with D&D, but perhaps with another system. Rereading Beowulf, I was caught up by the fact that magic and spells are mentioned, but generally only used by non-human entities. (Grendel has spells on him that make him immune to normal weapons, and Beowulf kills Grendel's mother by using a magical sword crafted by giants.) Perhaps dwarfs and giants can only enspell metals or earth... a dwarf couldn't summon a demon but could potentially make a magical hammer or axe, or perhaps magically sealing mithril gates for his clan's keep. Perhaps dwarves and giants use similar magic, which might account for some of the enmity between the two races. (Did one steal the magic of the other?) One of the very, very few things I liked about 4e is that all the monster races had idiosyncratic spellcasters... goblins had spellcasters who used hexes and curses, whereas hobogoblin war mages attacked with blasts of force, etc, etc. Of course, all monster magic was effective only in terms of combat, but I liked the basic idea.
I won't be changing the magic system for my current campaign, but it might be time to fill in some world details a little more. I find that I like this blank slate approach more than my usual "got to map it all" approach.