FNV has lots of "Classic Fallout Weapons" and other similar item mods, I would say go for it.
(I also suspect that some people aren't adding those because they would be hard be hard to justify being on the East)
Some might fit pretty well, like the Service Rifle as a mid-game assault rifle used around the NCR. The Assault Rifle from FO3 (R91 I think) is a great design, one of the best Bethesda gun designs. Varmint Rifle would fit fairly well, too.
Laser RCW as a laser burst weapon using the shotgun/rifle sprite would be neat - Laser RCW would be perfect as a weapon for the Salvatores in New Reno, Laser Tommy Guns fuck yeah. Imagine Salvatores fighting with Gatling-laser lites. Using a Laser resistances fix patch, it would be a pretty good mid-game gun too. Would be super cool for our NPCs too, imagine Cassidy or Vic with a Laser RCW.
One variety of item that would be really cool to see are all the unarmed weapons from FO3 and FNV. Also the ones from FOT too, but I guess they are already in. Thermic Lance as an end-game spear, fuck yeah!