Magazine Repacking
Magazine Repacking was introduced with 1.9.0.5. I think this is a great feature, it's always been a bit annoying having a bunch of half mags during a session with no way to fill them outside of arsenal or crate access.
While I think this is a good feature, it feels a bit basic. I actually opened my game when I saw the changelog and was genuinely confused not to find any loose ammo types in the arsenal to equip. I was a little disappointed to find out it was a simple mag merge system, but it's a start.
I think Arma would benefit with an ammo repacking system with the depth that we already see in titles such as Escape From Tarkov, DayZ and Wardogs etc.. These games utilize the mechanic in three ways.
Loose Ammo (5.56, 5.45, 7.62 etc), Container (Magazine, Belt, Box etc) and the ability to Pack / Unpack.
A player is able to grab a 'stack' of loose ammo and drag it onto a 'container', allowing them to load ammo inside. This could also be extended into belts and boxes. In theory, an assistant gunner for an LMG (M249) should be able to unpack a belt of 5.56 and load those same rounds into a stanag magazine (M4 / M16).
I think this would give players a lot more flexibilty with their ammunition control especially for a title such as Arma where we really should start to see systems like this implemented as a milsim category standard. A few changes would need to be implemented, magazines would be retained instead of dropped (Unless a fast reload system is implemented later down the line) and icons would need to be made for each calibre.
Magazine Inspection
It would also be nice to have a keybind to be able to manually check a magazine for ammo count while it is inside the weapon via an animation. Yes, we have a UI indicator to display magazine capacity but for servers that use limited UI, there isn't really an immersive way to check ammo count outside of inventory access. This system could be expanded to include animations for chamber checks and chamber clears.
Dynamic Link
On the topic of belts and links, games like Bellum have introduced the ability to dynamically add rounds to a gunners belt as an assistant gunner. This would be more of an assisted reload without slowing the rate of fire so gunners can sustain fire over a longer period of time. A system like this could also open up the potential for dynamic barrel changes, giving assistant gunners more of a purpose in the field outside of just ammo bearing like we see at the moment.
Wouldn't really expect these latter proposals in Reforger due to the animation workload that would come alongside them, but it would be nice to see in Arma 4 or later down the road if possible.