I'm liking the Hitchhiker reference.
In any case, I'm more of a "manual do it myself" kind of a guy (complete with excel grids), but I can see some use on a tool like this, especially where the planning stage is concerned.
There's a tool currently available called StoryForm:http://save-point.org/thread-3595.html
It provides a sort of planning guide from writing down stories, ideas, map possibilities, characters, etc.
And then Nick Palmer has an excel Game Design Doc/Spreadsheet:http://www.rpgmakerweb.com/support/tutorial/rpg-maker-vx-ace
Maybe if there was a way to streamline this so people would not have to download multiple tools, that would be great. The biggest challenge most people face involve databasing and game balance, so perhaps if there was a way to spit out some unique formulas (for VX-Ace), that would also be great, or a faster way to calculate enemy stats based upon a player's current level, that would also make things easier in the planning stage.