With the news that Dead Island 2 has been moved back to 2016, you're probably worried about having nowhere to turn to get your zombie fix. Other than the thousands of other zombies games out there, I mean. Like Dying Light! With mod tools recently appearing, it's not a bad time to revisit the zombie-hopping open world game and see what modders have been up to.
The difference between night and day in Dying Light is like the difference between, well, night and day. At night, the infected are far more powerful and bloodthirsty, meaning your only two options for survival are hiding and running, and neither of those work for very long. Daytime zombies, though, are far more passive, usually standing in the street and not even bothering with you unless you run right up to them or cause some kind of ruckus in their immediate vicinity. The I Am Legion mod changes this, and while it doesn't turn day into night, it definitely ups the challenge by altering the behavior of daytime zeds.
First and foremost, it adds more zombie spawn points. Thanks to the mod, the zombies (and we're talking the general type of zombies, not the recently infected or specialized types) no longer just stand around in the street. They walk and prowl around, as if they're actively looking for you, or at least looking for someone. Once they spot you or hear you—and now they can at much greater distances—they become much more aggressive than they do in the unmodded game, and their group AI has been beefed up, meaning they're more likely to attack in packs from all sides. They also don't give up quite so easily if you simply run away, and even when you enter a safe zone, they won't forget you. They'll be waiting for you to come back out. Eek.
In addition to this, areas you've cleared of zombies repopulate much more quickly. Armed human goons are much less frequent, which initially sounds like it would contribute to a safer city, but really means weapon and ammo pickups are much more rare. Buying guns and ammo from dealers is tougher, too, as prices have been goosed up a bit.
The end result is a much more hostile and dangerous daytime environment, which will require you to keep to the rooftops as much as possible, plan your routes in advance, and keep moving! As you can imagine, this mod makes beginning a fresh game much more of a challenge. Good luck.
You can grab the mod from Nexus Mods or Moddb. There are two versions, The Ultimate Survivor, and The Survivor, the second of which isn't quite as hardcore. Installation instructions are contained in the download. If you'd like to donate to this modder, here is the link to do so.