All suggestions are based only on my subjective opinion.

Suggestions about weapons:

1. It would be really cool to see FN F2000 with FN Herstal FS2000 CQB tactical frame in game as possible new weapon for Security in name of F2000 CQB (with classic 400mm / 16" short barrel and without attachment rail on left side).
   Fire modes: Semi-automatic, fully automatic
   
   Most of attachments will be taken from MK18 CQBR (excluding Drum Magazine)
   All siderail attachments on the right side
   As "exclusive" attachments for this weapon we can figure:
   Recoil Grip (FN America FNH FS2000 Hand Guard)
   
   GL-1 Smoke / GL-1 HE (FN GL-1)
   
   ISM 1.5x-1x (FN America FNH FS2000 Cover & Sight)
   

2. AK-74, unlike M16A2 (Insurgents) & M16A4 (Security), does not have a different version for both factions.
   To be honest, the idea came to my mind while playing CO-OP:
   Playing with the AK-74 and AKS-74U as a Insurgent, I compared the experience of playing for the Security with the M16A4 and came to the conclusion that the bots of the Security doesn`t have weapons with magazines from AK in caliber 5.45 or 7.62 (yes, I often steal magazines from the weapons of the defeated opponents, they belong to me by right xD), while the Insurgents bots have weapons for almost any caliber and type of magazine.
   Security can get the latest version of current AK assault rifles series - AK-74M (for a greater difference from the standard AK-74 version standard stock can be replaced with a skeletal folding one & standart handguard can be replaced with tactical - LCT АК-9 handguard for АК-series, PK-299)
   Fire modes: Semi-automatic, fully automatic
   
   or AEK-971 (with tactical handguard - LCT АК-9 handguard for АК-series, PK-299)
   Fire modes: Semi-automatic, 3-round burst, fully automatic
   
   All siderail attachments on the right side
   As "exclusive" attachments for this weapon we can figure:
   Recoil Grip (Molot Vertical Grip Black 000186)
   
   Supressor (ДТК «ТГП-А»)
   

3. If you are interested in one of the previous suggestions about possible new Security weapons, then you should also add new weapon for the Insurgents, and it supposed to be:
   АН-94 "Абакан" as AN-94 itself
   Fire modes: Semi-automatic, 2-round burst, fully automatic
   
   Most of attachments will be taken from AK-74 (excluding Drum Magazine)
   All siderail attachments on the right side
   As "exclusive" attachments for this weapon we can figure:
   Supressor (ZenTac Airsoft ДТК-4 «Кочевник»)
   

4. It would be really cool to see several magazine-feed bolt-action sniper rifles for both factions. It will be middle-class weapon (not so expensive as M110 SASS and M99, but more expensive than M82A1 CQ and M1 Garand).
   For this type of weapon, I am considering the following models:
   Accuracy International AWSM as AWSM - for Security (chambered in 7.62×51mm NATO)
   Most of attachments will be taken from M110 SASS (excluding M110 Supressor, 6x-3x Red Ring, 7x NV Sniper Scope, they will be replaced by new one)
   All siderail attachments on the right side
   
   As "exclusive" attachments for this weapon we can figure:
   Supressor (Dead Air Armament Sandman-S Rifle Silencer 7.62 & Emerson Gear 22cm Airsoft Suppressor Cover Tactical Accessory EM9330)
   
   6x-3x Red Ring (Trijicon VCOG 1-6×24)
   
   4x NV Scope (ATN Gen 2+ Night Arrow 6-2 Night Vision Weapon Sight)
   Can be add as folowing attachment for: M24, M110 SASS, M82A1 CQ, Mk 17 Mod 0, Mk 14 EBR
   
   7x NV Sniper Scope (Pulsar Digisight Ultra N455)
   Can be add as folowing attachment for: M24, M110 SASS, M82A1 CQ, Mk 14 EBR
   
   СВ-98 as SV-98 itself - for Insurgents (chambered in 7.62×54mm R)
   Most of attachments will be taken from SVD (excluding SVD Supressor, 6x-3x Red Ring, 7x NV Sniper Scope, they will be replaced by new one); also its have an integrated bipod
   All siderail attachments on the right side
   
   As "exclusive" attachments for this weapon we can figure:
   Supressor (Zbroyar Monocore .308 Winchester silencer)
   
   6x-3x DOS (ПКС-07)
   Can be add as folowing attachment for: SVD
   
   4x NV Scope (Прицел ночного видения 1ПН93-4 «Магнус»)
   Can be add as folowing attachment for: AKM, AK-74, AKS-74U, AS VAL, SVD
   
   7x NV Sniper Scope (1ПН34 a.k.a. NSPU)
   Can be add as folowing attachment for: SVD
   

5. For a variety of shotguns in game arsenal, it is worth adding:
   Benelli M4 Super 90 as M4 Super 90 for Security
   Fire modes: Semi-automatic
   Capacity: 5+1 or 6+1 or 7+1
   Optics attachments mounts on integrated rail
   Underbarrel attachments mounts on special handguard (FAB Defense BM-4 Benelli M4 Polymer 4 Rail System)
   Siderail attachments mounts on the right side of special handguard (FAB Defense BM-4 Benelli M4 Polymer 4 Rail System)
   
   Franchi SPAS-12, stockless as SPAS-12 for Insurgents
   Fire modes: Pump-action or semi-automatic
   Capacity: 5+1 or 6+1 or 7+1 or 8+1
   Optics attachments mounts on sights mount (Aimtech Shotgun Saddle Scope Mount Remington 870 12 Gauge Black Matte or Magpul M-LOK MAG592 1913 Milspec Polymer Rail Section or SPAS-12 / Elite / Prestige Mounts)
   Underbarrel attachments mounts under handguard on special rail (Magpul M-LOK MAG592 1913 Milspec Polymer Rail Section)
   Siderail attachments mounts under magazine tube on special rail (Fyzlcion Tactical Tri-rail Barrel Mount For 12 Gauge Mossberg 500 & QD Sling Swivel Mount Rifle Accessories)
   

Suggestions about attachments:

1. Laser Sight attachments for pistols can have more authentic look, if they would be based on:
   NcStar NCS AQPTLG on HiAirsoft Pistol Rail Adapter 45 Pistol Under Rail Laser Mount COLT 1911 BK
   (For Tariq, Browning HP, M9, M1911)
   
   Umarex Walther PPK-S laser sight (For Makarov)
   
   NcStar NCS AQPTLG (For L106A1 & PF940)
   

2. Compensator / Flash Hider attachment for Tariq can have more authentic look, if it would be based on:
   Nine Ball Marui M92F Short Compensator (or at least give it model of Compensator / Flash Hider from M9)
   Standart Tariq compensator & FH looks really out of place
   

3. Compensator / Flash Hider attachment for L106A1 can have more authentic look, if it would be based on:
   Sig P226 compensator 1/2x28 Range18 RHT thread compensator
   

4. Compensator / Flash Hider attachment for PF940 can have more authentic look, if it would be based on:
   Dark Hour Defense Glock 17 Compensated Stand Off Device
   

5. Compensator attachments for some rifles, carbines & machine guns can have more authentic look (standart compensators & FH looks kinda bulky), if they would be based on:
   AKademia ДТК «Штурм» (For AKM, AK-74, AKS-74U, Alpha AK)
   
   "Вектор 7.62" ДТК VR-05S (24×1.5; .308 Win, 7.62×54mm) (For M24, M240B, FAL, ACE 52, SVD, M1 Garand, M110 SASS, Mk 17 Mod 0, G3A3, Mk 14 EBR, Tavor 7)
   
   Or SRVV AK FHMB Muzzle Brake for same rifles from above quote
   
   Ncstar SKS Long Muzzle Brake Pin-On (For Mosin-Nagant, SKS)
   

6. Extended Magazine attachment for Makarov can have more authentic look, if it would be based on:
   PMGrip Magazine Extension for Makarov pistol (+2 round)
   

7. Extended Magazine attachment for Tariq & M9 can have more authentic look, if it would be based on:
   Beretta 92F 9mm Magazine 20rd NEW PRO MAG 92 F M9
   

8. Extended Magazine attachment for MP5A2 & MP5A5 can have more authentic look, if it would be based on:
   M4 Spectre quad-stack 9mm magazine
   

9. Extended Magazine attachment for most of the rifles can have more authentic look, if it would be based on:
   Magpul PMAG 40 AR-M4 GEN M3 (5.56×45mm NATO) Magazine
   (For M16A2, M16A4, M4A1, MK18 CQBR, QBZ-03, L85A2, VHS-2, Honey Badger, AUG A3)
   P.S. capacity of extended magazines for rifles from above quote can be increased from 35 to 40 rounds.
   
   PUFGUN AK 5.45×39mm Quad Stock 60 round magazine
   (For AKM, AK-74, AKS-74U, Alpha AK)
   P.S. capacity of extended magazines for rifles from above quote can be increased from 35 to 40 rounds.
   

10. Extended Magazine attachment for SVD can have more authentic look, if it would be based on:
    or SGM Tactical VEPR Magazine 7.62×54mm R 20 Rounds
    

11. 6x-3x T Scope attachment from M110 SASS must be avaible for M24, Mk 14 EBR & M82A1 CQ
    (Its really cool looking and comfortable scope, but there is only M110 that can be equipped with it, pretty unfair for weapon with universal optics mount)
    

12. 1x KOBRA attachment for AKM, AK-74, AKS-74U, AS VAL, SVD can have more authentic look (original "Kobra" have a build-in side mount), if it would be based on:
    ЭКП-1С-03 «Кобра»
    

13. 4x PSO attachment from AS VAL & SVD must be avaible for AKM, AK-74 & AKS-74U
    

14. Replace AKS-74U current Underbarrel attachments handguard with better one:
    CNC Lightweight russian AKS-74U handguard - Black [5KU]
    

15. Recoil Grip attachment for AKS-74U can have more authentic look, if it would be based on:
    CYMA Real Wood SU Style Grip For AKM AEG Rifle
    

16. Bipod attachment for G3A3 can have more authentic look, if it would be based on:
    JG Polymer Handguard with Integrated Bipod for G3 Series Airsoft AEG Rifles
    

17. Bipod attachment for G36K can have more authentic look, if it would be based on:
    Begadi G36 Frontset with Outerbarrel & Bipod
    

Thank you for your attention!
I hope you like some ideas.