I would like weapons from both all three numbered Fallout games and ones we haven't seen in Fallout before (this can be both real life weapons or fictional ones.) After all, all of the Fallout games are set in the same universe, so it only makes sense that guns which exist in one game would exist in others. Besides, variety is nice, so the more types of weapons they can conceivably put in the game, the better, and if the designs used in the game have already been used in previous games, it's a bit easier for the graphical designers as they don't need to design completely new weapons, they just need to take weapons that have already existed and create models that are high quality enough to be used in New Vegas. Maybe for Fallout 3 weapons, they'd even just use the same models, it's hard to say if New Vegas will actually use assets from Fallout 3 or if it's just the engine that was copied.
Of course, I highly doubt Obsidian will put every single weapon ever shown in the main series into the game, so it would just be a mixture of weapons from each of the games, using whichever ones the developers prefer, or feel would fit better in the game.
Technology advanced in a different path then our time line and they did make many advancements in weaponry, but that does NOT mean any suggestion for a weapon that was made after 1959 is a no go. Must I remind you Fallout 1 had the desert eagle and fallout 2 included the P90, XL70E3, H&K CAWS, Pancor Jackhammer, and the M60.
Of course, this only means there's a precidence for such weapons in the series, and despite what some seem to think, just because an idea was in the series from the start doesn't make it
good.
However, I don't object to the real weapons made after the '50s in the earlier Fallout games, and I wouldn't object if they came back. It's not like the split means that every weapon ever developed after 1959 can't possibly have existed in the Fallout universe, besides, designing convincing fictional weapons to fill the niches of real weapons that look like they could actually serve the same function and don't look like they were designed by someone who knows nothing about guns and at the same time don't look like they're just copies of real guns by a different name is hard. Using real guns would be easier for the designers.