In my opinion, a big problem with unturned 3.0 is the amount of loot. If you play singleplayer you can find a town or campsite really easily and get enough food and drinks to last days.
In 2.0 this wasn’t as bad because a lot of food was mouldy and the houses were smaller so there is less loot.
And in 1.0 you had to walk for minutes on end just to find a building, which was normally almost empty.
So I think, in 4.0, you should have a bit of a combination of the two; massive maps with space buildings and towns and mouldy food.
It would put an emphasis on hunting and farming because in 3.0 these features are barely used.
Other ideas:
Darker nights
Free roaming zombies
Guns tend to jam
Ammo is rare
More melee weapons
Larger maps
Head bobbing when running
Doors and windows on houses
Having to breaking into a house via windows to loot, also this could attract zombies.
There isnt much information about the zombies/turned as of yet but those have been suggested a couple of times, so far we are in the dark. Alot of these are pretty common sense to add (food spoilling,rare ammo,hard zombies,) although guns jamming is a fairly hot take, and you can read up on what other had to say on it on other posts like this one: [Realistic Gameplay] Real gun durability and jamming
I mean darker nights is a pretty common sense idea, and with night clearly not mattering in 3.0 Im sure nelson must have had the idea bouncing around in his brain before you suggested it.