I'm going to go ahead and say it: I liked Fallout 3 AND Fallout NV and part of why I liked them both was how different they were while maintaining a similar-enough baseline that I felt at home in both.
Now, maybe that's because I came into the series at Fallout 3 and hadn't played any of the other games that came before - never had the system, the time, the interest, the money etc. - and so my view may be skewed by that fact, but my view is still valid because I'm sure there are plenty of people who came in at that point and not before as well.
I'd like to see wasteland - there should be wasteland, the timeline doesn't matter as much to me ("realism" is a stretched term in a game of such a theme honestly) as the thematic idea of the game being "post-nuclear-apocolypse." I'd like to see more of the factions we've seen so far and some new stuff. I want to feel like its familiar but has a new twist here and there. I like a change of scenery from time to time in the DLC, but for the main theme I do honestly expect to see bombed out buildings, shack survivor towns, odds and ends of those who were there before, and all the rest. That's a huge part of the gameplay for me - uncovering the pre-war relics, stories, information, clues whatever you want to call them.
I'd also love to see a re-make of the first two games - an update perhaps or a remastering - for those who missed out on them, but that seems less than likely with what I understand to be the studio changes and rights purchase transfers and things that have happened in the real world relating to the franchise.