I think the future setting of Arma 3 has spoiled the game a little... i think the people want weapons (as well as vehicles and aircrafts etc) which they are familiar with. The time should be set between WW2 and today (at maximum), but nothing too crazy like space, or 2050 or something.
some other thoughts..
-More detailed map (instead of going just bigger and bigger)
as in most missions the battlefield will be limited to a certain area of the map anyways in most cases.
The variety of buildings should be bigger, it was already a nice step to have every building entre-able, but i think this doesnt have to be, i'd prefer to see a greater variety and bigger buildings even
-More gameplay features
i.e melee combat, even just to increase the modding ability!
Other ways to overcome obstacles, like jump (but still keeping it realistic).
At least an animation when you fall down from some heigth!!
Better ways to take cover, into the direction of GTA5.
Maybe a possibility to change the style and speed of stances, like from "at ease" -> "stealth" -> "combat" depending on the situation the player could change this manually.
Or maybe even automaticly, when you have your weapon lowered -> "at ease", if you raise it -> "stealth" and if shots are fired nearby -> "combat",
because i guess the least ppl do the work of pressing the C and CTRL+W/S to change the stances, its nice though when making videos for sure!
Also i think you should revieve the good old Warfare gamemode! Building up your own base, recruiting units, capturing cities to increase your income, thats one of the coolest things ever in this game.
And an AI commander, so you are able to play it even Coop!