Man, now I really want to run a sandbox game.
Get a big wilderness hex map, fill it with cool stuff, and be ready to add things as the players start moving around? Does that ever sound awesome. Throw in a few huge, nonsensical dungeons (one big dungeon with entrances all over the place! woohoo!) and I am totally sold.
I have never, ever done anything like this. Never run it, never played it. But I really, really want to try.