These are relatively lower-impact features that I think would add some flair or quality of life to the game.
Vehicle illumination
I think there are a couple of recent and planned changes that would already go towards supporting this. A major one is the change in art direction from full military towards more of an armed hazmat response, which I think this suggestion would boost massively. Another thing is if we're getting more space for command buttons then that would allow for one or more toggles for the functionality.
The long and short of it is emergency light bars on at least some vehicles. With LED tech these have become extremely configurable over the years and there are some things that could be replicated ingame that would work great for us. There could be a white and amber flash pattern you can turn on just for ambiance, but more practically, there are front takedown/flood and side alley light functions to light up the scene. See here: https://youtu.be/y4jn7Ak6UoY?t=132
I think the Springboks and Humvees would be great candidates for this, though maybe not some of the heavier equipment.
Cosmetic customization settings
I think a checkbox that would make it so any given vehicle you buy has a, say, 25% chance of spawning with the radio turned on and playing a random track from your library would be fun.
Another possible one would be a tickbox for vehicles (and later infantry) to spawn with a random skin.
And then I'm guessing this is maybe planned but renaming vehicles, besides being just fun, I think would be very practical as well, like numbering recon drones to know which ones are patrolling and which ones are free to use.
Full squad loadout
There is currently a loadout system for individual units, but I would like to be able to save multiple units that I can then buy at once. I.e. if my standard squad loadout is 2 engineers, a medic, a sniper and 2 heavies, I'd set those up, save them as a loadout and then be able to buy a set of them with one click. Ideally this would be saved separately from playthroughs so you don't have to redo it each time.
Persistent patrol routes
I would like to see a system like Prison Architect where you draw a patrol route once and then assign or unassign units to it rather than having it reside in one unit's memory. It's a massive fuss to set up a patrol of a large area just for it to disappear if I need to refuel the unit or it gets killed.
Reorganized save/load menu
I would like the save menu to show playthrough 1, playthrough 2 etc and then once you click on one of them it would present the auto and manual saves that belong to that speciifc run. As it stands things get all mixed together and it's not great to navigate.
Minigun version of the CROWS turret
Brrrrrrrrr for Springbok and Humvee.
a vehicle minigun without the blocker of having to animate a dude operating it
Resizable UI
We used to have this but I can't find a way to do it anymore as of the last UI revamp. I stand to be corrected. On a larger screen like 32-42" 4K the elements are just unnecessarily big and take up space.
Building blueprint system
Imagine you just laid out a nice little FOB with some fences, a guard tower, a generator etc. You do a selection box around it and save it as a blueprint that lets you smack down another set of those buildings elsewhere in the same orientation with one click. Examples of games that have a system like this are Supreme Commander and Anno 1800. Again as with loadouts these should be stored independently from individual playthroughs, more as part of a profile save.