This is just my opinion, but both nuclear and zombie apocalypses are somewhat...Common.
I fastened on two words in the original post: Roadside Picnic.
For those unfamiliar with it, this is the Russian novel on which the Stalker movie was based. The idea is that the aliens came and left...And didn't bother making first contact. Instead, they contaminated vast areas of the earth which now are called 'Zones.' In the Zones, the laws of physics may not always apply. Alien technology is so strange that an awful lot of it makes no sense whatsoever...Imagine a stone-age man trying to fix a car with a bad sparkplug. Even if you explained what part was wrong, and tried to explain how it worked, the best he could probably do is try to whittle something out of wood which fit into that space. The worst might be misunderstanding how 'spark' was applied and setting the car on fire, possibly with catastrophic results. The Zones had strange effects on the children of those near them... And sometimes on the dead, as well, if you must have zombies.
So how would others feel about a situation in which there was a total breakdown of society, a massive population reduction not due to fear of plague, but due to fear of what one's own children might become, strange and monstrous beings, bizarre technologies, most of which were not understood at all (in the books, humans were able to work out that certain things could be used as batteries, and others were effectively sonic grenades, but that's probably not what the original owners used them for).
The title of the book comes from one scientist's hypothesis about why the aliens didn't make contact, and why they left all this stuff behind them contaminating the world: if you stopped by the side of the road for a picnic, left your trash lying around, and moved on, you wouldn't bother to inform the ants, would you?
Note that this could be mixed in with the nuclear, plague, and zombie apocalypses. All it takes is a few major nations trying to cleanse the zones with hydrogen bombs, a few alien diseases added to the ecosystem (there were hints of these in the books) and the walking dead are already a part of the setting.
Anyone have any thoughts on that?
I'll also note that one interesting and different post-apocalyptic group misfit-and-outcast idea would be a small travelling "gypsy" (using the word to describe a paradigm, not meant as a slur) carnival/freak show (see previous). These things were all the rage during most of recorded civilization, and it's only in recent years that they started being large enough to require trains to carry them around or large enough to have permanent bases of operation. During most of history, they rolled into town, tried to swindle and scam goods and funds out of the locals, then rolled out again before they ended up in jail.
Well, before MOST of them ended up in jail.
Just an idea. I'm not married to either of these ideas (my husband is too snuggly), but I thought they'd be interesting to chuck out there.
And, as an aside, the classic distinction between a heathen and a heretic: they're both externally-imposed ideas -- the heathen may be forgiven his ignorance of the righteous ways, but the heretic knows the truth and tries to deny/fight against it. In the classic Inquisitional sense, it's the difference between an idolator (too many of them, why waste our time) and a Satanist (you know who the Enemy is, and still you choose to follow him!).