The newbies to the Fallout series HAVE to stop clinging to Fallout 3.. Believe me you won't care about the capital wasteland once you get a taste of the western climate.
It seems like everyone clings to the first Fallout game they play, even, or especially, our esteemed fellow fans at that
other site. But yes, new fans, there is much more to the Fallout universe than what is in FO3 - think outside the box. Go play the originals at least once, or barring that, go to the Vault Wiki and read up on them in detail. You'll find a lot of fun and exciting ideas that make you look at the Fallout universe differently. The Fallout Bible is especially good reading. It has information not given in any Fallout game, but written by Chris Avellone, who has written New Vegas. You might get some insight into how he thinks.
Seems a bit premature to be thinking about expansion ideas since I don't even know what I have yet in New Vegas, but I'll bite. Dogtown seems awesome, and I would love to explore an area of ruined skycraqers. Regardless of the DLC, I want expansions that give me more area to
explore, like Point Lookout. I don't want run and gun corridor shooters like Operation Anchorage or Mothership Zeta. I enjoyed the Pitt because it was such a great environment and had a good story and some neat, if constrained, exploration, if only in a limited area.
An interesting idea for DLC are what I would call, Tweak Packs. DLC similar in a way to Broken Steel that didn't add new areas, but modified the ones we already have. Ones that introduce additional quests and NPCs to the main world and environment, made existing quests more elaborate, added new casino games, or added another hotel on the Strip for instance.
I would also like to see radio DLC. Radio DLC could expand song lists on existing stations, possibly add new banter and DJ quips (if they can get the voice actor back in the studio), or even add new radio stations all together.