TL:DR Better AI, a bit more tools for stealth, change weapon crates to a better system. More control over the way you want to play a sandbox game.
Really I am bored waiting for the game to come out, can't wait.
I think I'm going to enjoy the game but I also think their are simple improvements that could be made.
Being able to move bodies. I would love to have this be a more open ended Splinter Cell, for stealth play, or Trap bodies for going loud.
A more permanent world, if I blow up a sam site they shouldn't repair it as soon as I leave. Longer range sniping. Being able to blow up outposts to slow or lessen the reinforcements.
Some kind of crosshairs on the helicopters so I can shoot rockets or miniguns with some effectiveness.
Being able to suppress all weapons, I like the Noise reduction meter just let me suppress every weapon with screw or clip on silencers, then you can make the flash hider/muzzle brake/compensator choice matter. Flash Hider makes it even harder to detect where the shot came from, muzzle brake does more damage, compensator does less recoil. A formula like that could allow for a choice while promoting stealth co op.
No Weapons Crates, get evidence through intel/interrogations, that count towards parts and weapon unlocks, you get through supply drops, that could be shot down by sam sites. You choose what weapons you want.
More options in gunsmith, I don't mind if they are cosmetic just would like to see a good selection of customization.
More control over your squad. I'd like to name, dress, equip my squad, and have it matter a little, if I give them long range weapons they can sync from farther away, short range weapons they are better in close.
It probably won't happen because of consoles, but I would like more control over my squad. I'd like to split them up, Have them drone, or provide sniper cover, suppressing fire, or just let them drive/fly.
Better Enemy AI, it is always wanted just a given. I don't like the AI Knowing where I am without them seeing or hearing me, and unless they communicate over the radio, or yell the other enemies shouldn't know either.
Make it sensible about surrendering or interrogations. I wouldn't mind if the AI had a morale system where I could scare them to run or not fight. For instance if I grab a high ranked Nacro stuff him in the trunk and park it by a patrol they should freak out. Especially if I throw C4 in with him.
IEDs, guerilla warfare, hell I wouldn't mind being able to convert enemies to the rebels cause. You see alot of my ideas in other games, but I would prefer Ghost Recon being the best at this kind of gameplay.
Also I would like a knife for stealth melee kills.