The problem: Both Oblivion and Fallout's take on underground exploration was pretty dull; recycled environments are not what you want to be witnessing 3 hours into a 100-hour game. Although Skyrim had a far more diverse array of caves, mines and other hideouts there was still a hefty amount of reused material. No game of that scope could possibly make every interior completely unique - but it was a bit too soon before we noticed the same few caves constantly popping up.
The solution: We doubt Fallout 4 will bask in the same kind of beauty that Skyrim's underbelly often displays, but the huge increase in design variety is something Bethesda can use moving forward. Most excitingly, we love the idea of having more vaults to explore. Fallout's back-story is nothing short of fascinating, the scientific and social experiments that were performed on the general population are only touched upon in Fallout 3 and New Vegas.
As well as letting us interact with more factions on the surface, why not introduce us to entire vault colonies untouched by the radiation that has enveloped the world outside? We can't help but think the Fallout universe's surface has barely been scratched yet.