Hmm, it is quite a lot of work.
My project is pretty sandbox-y. It's sort of like a more linear Oblivion. Scratch that, it isn't very linear at all. I have a main quest, with a bunch of side quests. After a certain part in the main quest you can just completely forget about it.
It is a lot of work in the sense of Switches and Variables for all of your choice needs, the maps and their consistency as most sandbox games don't have world maps, and enemies! Random encounters in areas won't work unless you have a leveled enemies script or something...or else the game still feels linear as you'd need to be a certain level to go to this place. But it shouldn't be oh so leveled. I don't know, it's pretty hard to explain.
But yeah, it can be done, and it'd be interesting if it get's done. Hopefully I finish my project one of these days/months/years ._.