Pretty sure they are adding a BASH (rifle butt, etc.) when wielding a ranged weapon. This would be a perfect tool for a perk that provides some knockback, stun or even knock down type of mechanic (like the take down move from NV). Not sure a shield really fits into the Fallout Universe. Now as a mod, this might have some potential.
Maybe those eggheads at the Commonwealth found a way to make a portable photonic resonacce barrier: http://fallout.wikia.com/wiki/Repulsion_device Something like what the gunguns had here at 1:40(activating) and 2:48(in use): https://www.youtube.com/watch?v=p6R9UkpyvSM
@ OP
Riot shield is actually an excellent idea that would allow them to introducing a sword and shield like playstyle. Useful for melee vs melee AND ranged...it would have to be some kind of high-tech riot shield. Either a forcefield like device with a battery charge or some kind of composite material that can take melee hits, gunfire and energy weapons fire but is meant to be repaired or replaced after prolonged use.
http://www.cliftonroadgames.co.uk/products/miniatures/privateerpress/images/PIP33007_man-o-war-shocktrooper.jpg...nuff said
That's already been released, it's called Skyrim
Exactly. Shields could even use microfusion shells or EC charges ammo for a temporary stun effect that can knock back enemies, allowing you to at least stand your ground and not run backwards from enemies.
The shield wouldn't be an 'I-win' button as they would deteriorate over time. Though a great addition for anyone role playing a melee warrior, who feels ranged weapons are for cowards. A bladed bat or a blood smeared machete in one hand, a stun charged, riot shield in the other.
Yah its missing mutants, miniguns and that whole post apocalyptic feel though.