*Magpul UBR/CTR
*Surefire 500 series handguard
*Swiss 551 handguards
*Aurora diopter
*Samson folding FS
*Sig/LPA 556 BUIS
*Swiss bayonet adapter
*Ace folding mechanism
*B&T/Samson/SigSauer quad rail
*Lancer L5 magazines
*A few coupled Sig 556 magazines
*Sig Sauer 556 sling
*Blackhawk SWIFT sling
*Blueforce/Spector/Blackhawk single point sling/mount
*Surefire,VLTOR,A1,Vortex,AAC or 551 replica FH
*Surefire "Scout light"/X series/TLR1 or 2
*Tangodown VFG
*Magpul "enhanced trigger guard"
*Griffen Tactical "modified" charging handle
*Zeiss Z-point
*Aimpoint Micro/M3/M4
*Trijicon Reflex/Accupoint/ACOG
*Leupold Prismatic/Mk2/Mk4
*EoTech 512
*Insight ISM-V
These are a few things that I would suggest looking at for your Sig 556. Some/many of these things I have tried personally.
Currently, I have two Sig 556's an ER and SWAT model. The ER has a Magpul UBR, Surefire 500 series "upper" handguard, 551 lower handguard, Blackhawk "MP5 style" single point, Swiss bayonet lug, Swiss Arms N+ FS and 1913 diopter.This gun has an EoTech 511. Sadly, I have enough $$$ in accessories that I could have easily bought another 556 or two... FWIW, the Swiss Arms diopter is a colossal waste of 300.00+....
The SWAT model has the factory SigSauer quad rail, non-folding CTR assembly, factory "552" emergency sights, Leupold prismatic sight, Tangodown VFG, Surefire X400 and Blackhawk SWIFT sling.
My advice-ditch the abysmal folding mechanism or spend the minimal time/money to renforce it. Even reenforced, it won't ever be an LMT or ACE SD extension...The gun is already "porky" so be careful adding stuff to it. Add the accessories you need, but keep it as simple/light as you can...Seems like an oxymoron, but you can have a "lightweight" gun that has an RDS and weapon light. Leave the factory "emergency" sights alone. They are way more durable than they look. Don't use an EoTech unless you have a quad rail or Griffin charging handle.
Have fun with a great rifle...