We have the Technology, we can expand upon what is now established. We can build it bigger, faster, stronger, better than it was before.
"Making Fallout 3 was the hard part, going from Hack and Slash adventures to a Post Apocalypse adventure shooter was certainly no easy task. Bethesda should be commended for doing as well as they did with such a monumental undertaking. Now that the engine is up and running and Bethesda has the template, expanding on it should be easy in comparison."
Here's a list of suggestions for a Fallout 3 expansion or for Fallout 4.
Quests should ideally effect the ending of the game and have multiple possible solutions including Non-Violent solutions for most of them.
Your main character should develop a reputation for his actions in various towns.
There should be more quests.
NPC characters should be more interesting, and there should be more dialog options. Companion NPCs should be made much more interesting, have more conversation options, as well as quests.
There should be more potential companions, you should be able to recruit more of them (up to 3 at a time, not counting Dogmeat or exploits) and you should have more control over their combat tactics and inventory.
Evil Dialog choices should be more evil and less agnsty and rude, and they should change the way NPCs react in a meaningful way. Evil dialog should not amount to mere dismissal of the NPC's thoughts with pointless insults, but should also include more persuasive, appealing, and often sinister options. There should be more appeals to a player's pride, greed, envy, and lust for evil characters, with options reflecting these things. Currently most options appeal to simple spite, with a few appealing to greed.
The Main Quest should be less confining and make more sense, with a clear cut goal and a less ham handed execution, the Villains should also make more sense, and not just self destruct absent of some really persuasive arguments.
S.P.E.C.I.A.L. Attributes should be more meaningful, and so should skills and perks.
The towns should be more interesting, including bigger towns with a stronger criminal element and more potential faction/karma/skill specific quests.