The problem: Bethesda games are always incomprehensibly massive, but Skyrim raised the bar. Both Fallout 3 and New Vegas felt relatively slim in comparison, offering little in the way of variety. There are clearly limitations to what you can include in a world supposedly destroyed by the fires of nuclear war, but we think Bethesda can still do more to please our eyeballs.
Skyrim had 9 cities, all of them varying in size, architecture and style, while Fallout 3 had only the decrepit downtown DC with smaller settlements dotted around. New Vegas was similar, with only the disappointingly small, illuminated Vegas Strip to stand out from a few minor towns spread across the desolate environment.
The Solution: Fallout 4 needs to display visual and cultural variety on the same level as Skyrim. Let's see interstate exploration on a massively detailed scale, where the kinds of variety seen in the Fallout 3 DLC packs are playable in one seamless wasteland that stretches on for miles and miles. Have multiple cities spread across the landscape that offer up hundreds of quests and activities for us to partake in, and have them vary enough in their destroyed state to warrant our exploration.
Of course, with more cities and settlements comes the opportunity of politics and other post-apocalyptic pickles. The nuclear dilemma of Megaton and its potential detonation was a standout moment in Fallout 3, but it felt more like a scripted set piece than an organic event you simply stumbled upon. Fallout 4 needs more decisions like that as well as bigger consequences for it to match the bars set by its predecessors.