According to Jessen, the co-op gameplay is the core mechanic of the game. It was "built from the ground up as a co-op game" as friends have to scavenge for supplies and build fortifications during the day to withstand the undead horde at night.
As such, there will be a day/night cycle in the middle of "semi-procedural environments." Meaning, the game won't be completely random.
Furthermore, while the game resembles Minecraft in gathering resources, the action is pure Epic Games, and both parts of the game will work in concert with each other. For example, weapons are upgradeable. While the standard crossbow will only fire one arrow, it can be upgraded to fire three at once.
The game will also feature more than just zombies. Trolls, for example, can climb over walls and sometimes even dig under them, while stealing supplies.
Cliff Bleszinski also promised that Fortnite was a PC game first and foremost. He did say there might be a console version "somewhere down the line".