There are some amazing roleplaying settings out there that haven't been officially tapped for creation. Or, occasionally, they've had a game out in the past but it's fallen into disused or unsupported territories. In either event, sometimes I feel the need to talk about what I'd do if I had a chance to write the book on how to play in these settings. Typically, this will be as a conversion to Fate Core (as that's my current system of choice) but that may change if I change systems and on occasion I may run with a different system that feels appropriate.
I will try and avoid spoilers in what I write here, but the very nature of trying to build a world for gaming in is seeing behind the curtain a bit, so there will be some spoiler-y material. You are forewarned. If you don't want to know, now is probably a good time to stop reading this post.
Okay, that all out of the way, the first setting I want to talk about is the world of Mira Grant's FEED, which is the first novel of the Newsflesh trilogy. In FEED, zombies have finally appeared and spread throughout the world. Civilization has recovered in a way fairly different from your typical zombie movie. Instead of devastation marauding gangs, and lone survivors, civilization fought the zombies back, despite the intense contagious nature of the disease, and there's still power, the internet, TV, university, government, and plenty of other normal, everyday features of the modern world.
The plague is brought on by the Kellis-Amberlee virus (which isn't a spoiler, as this is well-understood by all the characters in the book), and the characters are regularly blood tested (using small hand-held machines with green "safe" lights and red "dead" lights) to see if the virus is spreading in their bodies ahead of their conversion into a zombie. Every mammal is infected, every mammal over 40 pounds becomes a zombie or "amplifies." Amplified zombies are fast at first and slower as they age (and decompose). Zombies alone are fairly stupid, plodding creatures, but they often moan to attract the attention of other zombies. As the zombies collect, they gain a sort of hive mind, getting smarter and attempting to lay ambushes, find weaknesses, or create distractions. The characters in the novel are also aware that the virus drives zombies (and zombified dogs, cows, deer, horses and so on) to first bite victims to make them amplify and once the horde gains a certain level of intelligence to then find victims on which to feed.
The characters of the novels are bloggers, who have basically overtaken the standard press of our world/time. They are licensed and can move into more dangerous zones of zombie activity, which are rated 10 (basically as safe as possible) down to 1 (where you're basically shot as soon as you leave). Generally bloggers break out into three groups. Irwins (named for Steve Irwin) are the adventurers jumping into zombie infested areas and doing the dangerous things most people would avoid at all costs; they're always out in the field. Newsies report the news as factually as possible and do interviews as well as research pieces, and seem to take to the field about half the time. Fictionals write poems or stories based on the zombies, the world without zombies, or whatever catches their fancy; they're in the field extremely rarely.
I'll touch on what are called "reservoir conditions" just briefly. These conditions are when the Kellis-Amberlee virus takes over a body part of a host, but don't push over into full amplification. These conditions are very important for spoiler-y reasons, and it would be to the benefit of a GM to know why and if their players have one. It's also important to note that in this setting, the works of zombie writers/moviemakers such as Romero are considered visionary pieces which helped to save lives at the outbreak of the virus.
So that's the general setting. Now for some suggestions rule-wise.
Zombies should have a separate stress track which corresponds to the size of their herd and therefore the herd's intelligence. This stress track should probably increase one box with every five zombies, meaning 1-4 zombies would have a 1 stress box of intelligence and be fairly dumb, 5-9 zombies would have 2 stress boxes and come up with simple plans, 10-14 zombies would have 3 stress boxes and could potentially lay a basic ambush, and so on. Once the zombies reached 5 or 6 stress boxes, it would probably cap and they would stop getting smarter, lest we have zombie hordes piling into planes to fly off in search of more food.
I think it's not super necessary mechanically, but probably cool as a roleplay element if each player had an amplification stress track. Even better might be if the GM had a copy of it and the player had a copy, reflecting the character's true danger of amplification and the character's perception of that danger. This track would likely reset after each scene and could have boxes checked off by things such as extreme stress, exposure to zombie fluids, injury, blood loss, other illness, or other factors chosen by the GM. This might be too brutal a system for some games, but definitely would fit the story world.
The weapons and characters for this setting should basically follow that of a modern realism setting (of which there are several examples already in Fate Core, such as Fight Fire, Ellis Affair, or Crime World). It seems most fitting for players to play characters who are bloggers, but they could theoretically also be soldiers, police, detectives, scientists, or just hapless bystanders. There definitely should be a requirement that at least one aspect give them license to go into zombie infested areas (or a lack of license should be their trouble!). Aside from those things, a FEED setting should work relatively well under basic Fate Core rules.