Addressing the posts I missed ( I had to go to work before I could reply), there are some good ideas. I definitely agree that just increasing weapon damage furthers imbalance on other difficulties rather than balancing the game - most people do not play gold exclusively - I myself tend to stick to silver just because its easy and I don't tend to play with friends and often get crap teams on gold in PUGs. I do think some weapons need a little boost but certainly nowhere near the magnitude some people are talking about.
Whilst I agree that reducing enemy accuracy on gold is a less invasive change that helps weapons with sustained fire get more bullets on target, It will still take longer for the average AR user to kill that enemy than the guy who can just pop out of cover with his semi auto high damage weapon.
I do think your idea will really help, and I do think some weapons need a teensy buff, like the incisor, scimitar, vindicator, shuriken etc.
And the idea about giving rapid firing weapons more of a damage bonus against shields is good too, but it still does not balance it. Do you know why? Because even with reduced enemy accuracy, slightly more weapon damage and a shield damage bonus you are still absolutely screwed against two or three(at least) brutes, banshees, primes pyros, atlas's etc. if all you have is a piddly vindicator.
On another note - its not just about making the guns "viable" on gold, its making them so people have a good reason to use them on gold, because if you don't put them in a situation where the gun works better for them, they will all still just pick the same old gold weapons they always have.
Embellishing on the boost against shields idea, adding a threshold where any shots that do above a certain damage get a percentage based damage reduction to the shield mechanics would make it so low bullet damage, high ROF weapons are much more effective against shields. Sort of the reverse to the armour reduction where low damage guns get more of a penalty. It could fit with the lore well too, and be explained that higher velocity shots and larger slugs encounter more resistance when entering shields - like in the original Dune movie when Gurney Hallek is teaching Paul that a slow blade penetrates the shield, but a fast one does not.
The current shield gate mechanic is hardly a problem with marauders, geth hunters, centurions etc as many people just headshot with snipers, shoot them twice instead, or just use an anti-shield power. Atlas shields don't seem to go down especially quicker with AR's than heavier hitting weapons as the only damage penalty is applied to one single shot when the shield dissipates, because it's shield is so large it takes multiple shots from powerful weapons anyway and much less damage overall is wasted than say if you shot a marauder in the foot. So adding a penalty to the heavier hitting weapons nicely turns the tables on what has pretty much been a monopoly on gold.
Another idea I have seen around is the idea of adding a mechanic where the more bullets hitting a target in a short space of time, the more of a chance of the enemy flinching - sort of a suppressive fire mechanic. i think this would fit very well with the point of the enemy accuracy decrease idea - the enemies you shoot would be much less likely to shoot back. Albeit only the ones with no shield s or armour, but it adds to the general usefulness on all difficulties.
I think increasing the clip size and fire rate of most full auto AR's(and other rapid fire low damage weapons like the incisor and scimitar etc) rather than the damage would keep them more fitting to their original roles, and this coupled with the new shield damage idea would work well.
I know my idea (page 4) sounds overly complicated, but the premise is simple - it does not have to be delivered in the way I described it. Enemies could have grenade usage reduced, and any kind of wave works as long as there are lots of low health enemies. My idea is not necessary at all if the other changes that we have come up with are all applied, I was mainly partial to my idea because it also balanced power usage by making some underused powers better choices, as well as certain class powers like Ad rush, sentry turret etc and as such wouldn't just increase weapon variety but class diversity. I understand it sounds a bit over the top, but I would like some new types of waves to be added to the game anyway for fun so I thought of a way the ideas could all go hand in hand.