I don't know if i agree with the idea of a riot shield, first (as i have used in several occasions) they are cumbersome and hard to maneuver, second they ARE cumbersome, third they make you a larger target, fourth you are restricted to use only one hand so any weapon that requires 2 hands is out of the question, fifth they do not add much extra protection if any against projectile weapons, they only improve protection against a unarmored foe in closed quarters, and lastly did by any chance i mentioned they are cumbersome!!!! 
However with that in mind, energy fields are a whole different animal, and i do see those as being something that a great addition to the game. and i think i like Moriarty idea at the single side protection, how about that is implemented and further research will give you a full shield, i think that it should be made hard to research because of power issues and the need to be compact.
Yeah I'd want some good two-handed/one-handed weapon distinction if riot shields were implemented, as for instance the rocket launchers, blaster launchers, autocannons, and heavy * weapons seem to all be designed as two-handed. The other natural restrictions of riot shields can mostly be gamified by giving them a very high weight.
I'd LOVE a force fields mod though, that sounds really awesome, especially if it works very well against missiles, ie. blaster bombs, you could put it at an escalation point after blaster launchers so the aliens FIRST start killing you off with launchers, THEN they start bringing shields so you can't even use any of the damn things to kill them back. ^_^ It should be really easy to do a variety of sprite art for force fields, even with the restriction of having only binary transparency instead of an alpha channel for partial transparency, so we could have different types of "field projectors" that function in different manners, too.
