My favorite part of Fallout 3 was the exploring. Roaming the Capitol Wasteland, finding the oddball remnants of humanity or Super Mutant camps or labs full of psychotic clones, whatever. The fact the finding those places almost inevitably led to bloody conflict I frequently didn't survive was less enjoyable, but oh well.
Last week, it occurred to me I'd be really interested in a Fallout that was set in an archipelago. The Azores, the Aleutians, the Philippines, whatever. I think because not only could a lot of the islands serve as distinct settlements, but you could have fun with different creatures having evolved on each island in response to different outcomes from the nuclear war. Like Darwin's finches in the Galapagos, but run through 1950s sci-fi horror.
Here's an island with creatures made of sand. Here's a boiling island, because there's been some sort of seismic reaction that's got an undersea volcano going constantly. Here's a suspiciously pleasant looking island, that's actually alive. Here's an island dominated by cassowaries. Not mutated or super-intelligent or anything, just big angry murder birds.
Plus you have to factor in moving between islands, so that means pirates and merpeople (possibly friendly, if a bit strange to our eyes, or hostile) and giant sharks. Or penguins. Imagine trying to traverse the seas on a cargo run in whatever boat your character either found or built (if that's an option), and you've got that stuff to contend with, among other things. Maybe there's even an option for flying or gliding, if you can find a takeoff point high enough. But then you'd have to contend with whatever else can fly in the area.
There's probably already a game in that vein out there, but I don't know about it yet.