19. What single change would you make to a popular D&D setting and why?
My gut reaction to this one is to change the mechanisms of play rather than change the settings. I want to change the way spell slots work, and simply run with a magic energy pool that is drained by casting spells and recharged according to your chosen style of magical pursuit... but that's not what the question is asking.
My second rection is to change any one (or more) of the settings, and completely remove humans from it. I like what Dark Sun did to elves in the setting, I love the diversity of new races available in Planescape, I never really got into Eberron (because I had a falling out with the regular gaming group that I was a part of at the time when it was big, and never really got into a new regular group until it had somewhat subsided), but I'd love to have pushed any of these settings the next step further...making them a bit more exotic by simply pulling out the humans...either by a massive catastrophic spell effect or plague that wiped them out over a matter of days then playing with the resultant aftermath, by filling an alternate race into the niches typically held by humans, or by logically working through how the setting might have developed if humans were never there in the first place.
Perhaps humans are inextricably linked to the prime material plane, they can handle brief forays into planar regions, but gradually become weakened if they don't have a physical anchor (which gradually rots away as a sacrificial anode to prevent their own soul from being destroyed by the lack of prime energy in the realms of Planescape). Perhaps they need to wear cumbersome suits like those old diving suits if they want to survive for more than a few minutes away from the prime material... astral projection might work too, but that has it's own issues.
Humans are too prevalent...I understand why, it's generally because they function as a narrative anchor for the setting. A guage by which to measure the other levels of strangeness in the settings, but I think we've moved beyond the need for that in every single setting.