If you've read Darwin's World (rpg) I like the lore for it more than Fallout. In that game you have the Foundation (of Mankind) which has an origin much like the brotherhood of steel and even use a structure that includes 'paladin/scribe' but I like how they made the Foundation. I like the fact that in Darwin's World you have the Metal Gods. Robots who don't care for humans, and in fact turn them into cyborgs who work as their chattle.
They have the Brethern and the Brotherhood of Radiation, and many good elements of Fallout but done up in a way I only wish Fallout had originally been written in.
In this way I think Fallout is memorable in that, like cornerstones of pop culture such as Gamma World, (which influenced Fallout's creators) both of these influenced Darwin's World. Basically I like how these ideas feed into each other.
If I were going to create a new Fallout I'd use ideas from Darwin's World, such as an idea of the Metal Gods. Image a faction on the Wasteland and a particular vault where Machines Rule, and attempt to secure places like RobCo and creator more machines. I don't see that in Fallout but its a great additional idea, translated into Fallout universe (in that you'd use Mr. Handy, Robotron, etc.)
In Darwin's world they have droids which are creative thinking robots, ie like the androids in Fallout 3 but look like, well, machines rather than humans.