What I have made here is a very long list of every weapon, skill, attachment, and cosmetic I could think of that would be great additions to the game, as well as a few other gameplay items. Some of these are my own, but I have also added some frequently requested things I have seen in the community. I have been working on this for a while, so that is a part of the reason for the length.

Weapons

Assault Rifle:

XCR-M
HK433
SC 4000
AK107
SR-3M
G3A3A1
Groza
XM8
XM29
MSBS Grot
ASh-12.7
REC7
Famas G2
Mk16
ACR
LVOA-C
Mk18
MCX Virtus
M16A4

Shotgun:

M590A1
DP-12
XTR-12
590 Shockwave
590M
Origin 12

Submachine Gun:

PP-19
FMG 9
MX4
APC9
MP5SD

Light Machine Gun:

M240 Lima
CETME Ameli
HK21
Mk46
M60E6
IMI Negev
LAMG
MG3
MG 6.8
LWMMG
MG4
LSAT

Designated Marksman Rifle:

Mk20 SSR
L129A1
M110
WA 2000
Mk12
OTs-03 SVU
LMT MARS-H

Sniper Rifle:

M200 Intervention
Orsis T-5000
MRAD
AXMC
DSR-1
JNG-90
Omen Match 3.0
Steyr Scout
M40A5
XM109
MSR
SV-98
700 Tactical
Tormentum V
CS5
Steyr SSG M1

Sidearm:

MP17
Model 686 Plus
P226
PL-14
MP443
Skorpion
UCP
PP2000
PMM
Uzi
DroneGun MkIII (Will shut down the drone for 5 seconds and deal low damage. Can shut down generators from range without explosion)

Explosive:

LG5/QLU-11
XM25
M320
M79
AT4
FIM-92 Stinger
RPG-7
Mk 153 SMAW
FGM-148 Javelin

Special Weapon:

MCX Spear

Traits:
30 round magazine
Takes 6.8x51mm ammo, with a 250 round carry capacity.
Very high mobility, 700 rpm, stable recoil, and above average power.
Equipped with this optic, and will have a 2-6x toggle zoom.
Sight will have a rangefinder, with a small UI telling you the range your reticle is pointed at.
Mk Upgrade Perks:
Mk 1: +15 Accuracy
Mk 2: +3% Movement Speed
Mk 3: +25 Mobility
Ability:

1. Gains accuracy (10%), range (10%), and stability (15%) when suppressed. Only loses 15% damage when suppressed.

Unlock:
To counter the rifle being very powerful, it will be very hard to unlock the blueprint for. The challenge will go as follows.
1). Get each class to rank 5.
2). Kill 5 Behemoths with each class.
3). Get 100 kills with each class.
After those are completed, you will be given 3 side missions. Each one will have you infiltrating a heavily defended base, getting to a computer for intel, and going to a random Skell tech location with the crashed shipping container.
Upon completing those 3 missions, the blueprint will be available to purchase for 50,000 Skell Credits, and will cost 1,400 to craft.

Attachments:

Sierra 3 BDX (8x scope)
Trijicon MG Optic (5x with reflex sight on top)
Romeo H8 (1.5x optic)
Tango 6T (2-4.5x scope)
RMR Type 2 (Pistol reflex sight)
Razor AMG UH-1 (1.5x zoom optic)
Razor AMG UH-1 w/ Magnifier (1.5-3.5x toggle optic)
REAP-IR (4-6x scope)
Strikefire ll (2x optic)
Strikefire ll w/ Micro 3x (2-3.5x toggle optic)
D-EVO (1.5-4.5x optic)
ATN ThOR (4.5x optic)
SLX w/ Canted Sight (5x optic with canted red dot)
ThermoSight R-Series (2-4x optic)
TA31H w/ Reflex Sight (4x optic with reflex sight on top)
Pulsar Helion XP (5.5x scope)
BRAVO4 (4x optic)
Razor HD (4.5-7.5x scope)
ACSS w/ Canted Sight (4x optic with canted red dot)
Mark 4 HAMR (4x optic with reflex sight on top)
Leupold Mk6 (1-3.5-6x scope)
Leupold Mk6 w/ Canted Sight (6x scope with canted red dot)
Spitfire 3x Prism (3x zoom optic)
552 w/ G33 (1.5-3.5x toggle optic)
MGRS (1.5x optic with a wide view
MGRS w/ Magnifier (1.5-3x toggle optic)
NGAL w/ Flashlight
PEQ-15 w/ Flashlight
Covered Suppressor
AMTAC Suppressor (Overbarrel Suppressor)
Daniel Defense Vertical Grip
Coupled Magazines

General Weapon changes:

There will be an ammo type for every caliber, even if only 1 gun takes it. This will allow for more weapon combos that don't pull from the same ammo pull, and will add a fair bit of needed realism.
Increase muzzle velocity on all guns. Many guns have to be lead on moving targets even at close range, which should never be the case.
Remove the "after a kill" Mk upgrade perks. It puts guns that have them at disadvantages over those that have bonuses that are active more often.
Allow for elite traits to roll on Signature weapons. They are very limited by not being able to have traits like added mobility or handling.
Increase mobility for all guns. A buff of +40-60 depending on the weapon. The movement while ADS is very slow, even when using everything to increase it.
Fix magazine capacities. The DMRs should have 20 round mags as standard, many of the pistol capacities are too low, and the Mk17 and ARX200 should have 20 round mags as standard, with 30 in extended mags.
Improve attachment stats. Two attachments in particular are the RVG Vertical Foregrip and PEQ-15. The RVG has its pros vastly outweighed by its cons and the PEQ-15 only has 1 very weak stat and is beat out by the other attachments in the slot.

New Skills:

+15% Drone Evasion
+20 Mobility
+10 Handling
+10% Injury Resistance
+15% Drone Damage
+20% Range
+10% Bandaging Speed. +20% Injury Resistance for 15 seconds after bandaging.
+20% Body Pickup Speed. +15 Stealth and +10% Movement Speed while carrying bodies.
+25% Pistol Draw Speed
+75% Spotting Efficency and +50% Spotting Area with Binoculars.
Increase Binocular zoom to 6x.
Increased convoy loot
Longer combat roll (Improved distance and time to recover)

New Perks:

Add two additional perk slot skills, allowing for 5 to be equipped.
New: Sentry: +15% damage resistance; +20% injury resistance; +20% ammo capacity.
New: Stalker: +35% speed and +40 stealth while crawling.
New: E.O.D: +35% Explosive Resistance; +30% injury resistance from explosives.
New: Breacher: +40 Mobility and +15% Reload Speed with shotguns; +30% flashbang radius.
Buff: Increase "Close and Personal" to +35 mobility.
Buff: Increase "Sensor Hack" to +15% damage to drones.
Buff: Increase "Explosives Expert" to +30% AoE.

Items:

Separate Mk2 skills for every item.
When Mk2 item perks are unlocked, the Mk1 item will be replaced.
Increase Mk2 carry capacity to Mk1 carry capacity.
Breaching Tool will not need to be equipped. When applicable, the prompt and number carried will appear.
Mk2 Breaching Tools will be able to cut open doors locked behind enemy scanners.
Mk2 Binoculars will have a much higher zoom, spotting efficiency and area, rangefinder, and a clearer view. It will resemble an Oscar5 HDX Spotter Scope.
The Rocket Launcher will be a weapon and not an item.

Cosmetics:

Vest:

First Spear M.A.S.S.
Gunner First Spear M.A.S.S: First Spear M.A.S.S with 7.62 NATO ammo belts wrapped around the vest, along with weapon based magazines and pouches.
Spartan Armor Sentinel Legion XL (w/ side plates)
Atlas T7 Extended Coverage
Safe Life Defense FRAS
FirstSpear Strandhogg (w/ side plates)
FirstSpear Strandhogg w/ Hoplite Shoulder Plates (w/ side plates)
IBA (From Wildlands)
Heavy IBA (IBA with shoulder armor)
Armored IBA (Heavy IBA with groin guard)
Armored Wolf Vest (Wolf Armor with Riot Vest shoulder pads)
Armored Gunner Vest (Heavy BLACKHAWK S.T.R.I.K.E. w/ 7.62 NATO ammo belts wrapped around it, with the magazines based on the equipped weapon)
Walker Vest w/ No Cape
Tank Vest (The vest and extra armor the Los Extranjeros Armored units wear, with the arm armor, collar and groin guard)
Assault Vest (Similar to the vest on Heavy S.W.A.T from Payday 2)
Assault Vest w/ Ammo Belt (Ammo pouches from the Judge Plate)
Heavy BLACKHAWK S.T.R.I.K.E. (The Blackhawk vest with shoulder pads and a groin guard.)

Helmet:

Covered Ops-Core FAST Helmet w/ Mandible (Ops-Core Mandible-Sentry)
Covered Crye Airframe Helmet
Crye Airframe w/ Face Net (Danish Frogmen net)
ACH w/ Rails
Sentinel Mk.2 Heavy Helmet w/Mandible
Covered Team Wendy EXFIL Carbon w/ Mandible (EXFIL Ballistic Mandible)
K6-3

Shirt:

UF Pro Striker Shirt
Command Shirt (Shirt seen in the Sacred Land Pack image)
G3 Combat Shirt w/ Rolled Sleeves
Assault Shirt (Command Shirt with a kevlar sleeve on the right)
Heavy Armored Shirt (T-shirt with kevlar sleeves with elbow pads on both arms)
Fleece Jacket (From Wildlands)
Bomber Jacket (From Wildlands)
M65 Jacket

Pants:

UF Pro Striker Pants
Armored Sentinel Pants (Sentinel Kneepad pants with armor on the thighs, like a plain version of the Judge Pants)
Tank Pants (The pants the Los Extranjeros Armored units wear, though with similar styled armor added to the thighs)
Tactical Baggy Pants (Baggy Pants with Hill's Pants knee pads, a knife on the right, and Hill's Pants holster on the left)

Gloves:

SecPro Riot Gloves
Kevlar Gloves (From Wildlands)
Fingerless Kevlar Gloves
Mechanix Gloves

Mask:

Shemagh (A looser one)
Armored Collar (Plain version of the collar on the Judge Mask)
Armored Shemagh (Plain version of the throat guard on the Golem Mask)
Ballistic Mask
2/3 Ballistic Mask
Skull Balaclava

Patches:

IR versions of every flag patch in-game.
Rectangle versions of every flag patch in-game.

Camos:

Additional shades of green
Additional shades of black
Additional shades of brown
Various Tiger Stripe patterns
Various CADPAT patterns
EMR
Desert Night Camouflage

Skell Tech:

These will be new skills. They will be at the end of the branch that fits them best. Each of them can be toggled on and off, and will unlock a cosmetic based on it. They will each cost 5 skill points.
Hybrid Vision: Removes standard thermals and night vision. Adds Hybrid Vision. Unlocks Cross Com glasses. Hybrid Vision will be like Sonar Vision but with slightly dimmer signatures, constant enemy spotting, no drone disruption, and unable to see through walls.
Optical Camo: Activates when crouched or prone. Will decrease your detection range and rate. Firing an unsuppressed weapon will disable it for 5 seconds. Unlocks Optical Gloves, which will be Blackhawk Solag Gloves with tech built-in to them.
Reactive Armor: When taking damage from a bullet, there is a 10% chance for it to do no damage. 5 second cooldown after blocking damage. Unlocks a Talos set of armor, adding multiple pieces resembling the Kinetic Operations Suit.
Braced Boots: +5% boost to agility. Slight decrease to fall damage and injury chance from falling. Unlocks Braced Boots, which will look like reinforced Combat Boots.
Drone Detector: This will automatically spot drones within 150 meters. Unlocks Drone Backpack, which will be a Crye AVS 1000 with a DroneShield RfPatrol Mkll attached to the side and an antenna coming out the top.

New Class: Striker.

Technique: Override. Makes all drones within 100 meters go haywire. They start targeting other drones, enemies, and empty vehicles for 15 seconds, with reduced accuracy, and will overheat and take small amounts of damage.
Recharge:
Kills and damage dealt to drones.
Stealth kills.
Headshot kills.
Item: Jammer
When used, disables all lights, drones, and radios within 50 meters for 20 seconds.
Perks:
+50 Health.
+10% Agility.
+35 mobility with Assault Rifles and Pistols.
Level 10 Weapon: Mk20 SSR
What are your thoughts on these? There are a lot of things, but even feedback on a few would be great. If you'd like something clarified if it wasn't clear, I'd be glad to do so. Any other suggestions you would add I will add on to the list. Any typos pointed out I will fix.