I will just blast some ideas...
Chainsaw...
Fire axe...
Heavy plasma weapon...
Heavy electric weapon - something like ghostbusters (movie)...just with standard fallout smoked flesh...
A long shot for fallout universe but how about Liquid nitrogen cartridge for flamer or a grenade so when you freeze the enemies they burst into frozen pieces of meat...
Beretta 93R...
Dragunov sniper rifle...
Milkor MGL...
Sawed-off Browning O/U...
PK 7.62 mm general purpose machine gun...
Small guns flamer (like M240 Flamethrower )
M80 Rocket Launcher...light - simple - one time use (no need to carry a heavy rocket launcher)
Plasma rocket...
Maybe ?? MG 42 universal machine gun from WW2 - strongest machine gun ever made, and it still is...compare it's rate of fire with any machine gun...still used today...somehow I think it doesn't fit Fallout ??? maybe it does ???...(M56 Smart Gun was designed from it..so you all remember the sound...ohhh makes my skin crawl...)
A link to some weaponry http://www.dreadgazebo.com/gunporm/?cat=9 check the site not much but still some kick ass weapons... I hope that we'll see some of these weapons in the game...uh uh...
50 cal. sniper ammo with explosive tip...
Keep old weapons from Fallout series...like Browning M2...Bozar..etc..
I hope we'll be able to use two one handed weapons...like 2 pistols or Uzi...
Its better to live 3 days as a Lion, then spend the rest of your life as a hyena!