Well, the idea of a world without an extreme proliferation of extraneous level 1 assholes isn't outside the realm of creditability at all. One of the Tomes might have gone over this, I'm not sure, but I'll say it here anyway: That's what our own world was like, at one point, before agriculture made it possible to support the caloric needs of a large and sedimentary populous. The nomadic hunter-gathers who predated the occupation of all the nicest land by settled societies enjoyed better nutrition and practiced a more diverse range of skills than their successors. They also lacked a lot of the standing facilities to keep the sick and infirm around for very long, or support more than a single infant per mother at a time. You'd take an average member of their societies over the average member of the earliest farmer societies any day of the week. Thing was, though, that there wasn't just one farmer per nomad; there were
two hundred farmers per nomad. The original agrarian survival strategy was built on overwhelming numbers. Specialization was merely an emergent technique on top of that.
So, your undead plague will undo the circumstances that make it possible to support a vast and under-extraordinary population. But where the chief limiting factors in the population of a pre-agricultural society were the means of self-sustenance, the chief limiting factors in your post-plague society is going to be the means of self defense. Anyone powerful enough to personally provide for the protection of people other than themselves is going to find their capacities saturated by the up-and-coming generation, so anyone else that can hack it in adulthood is also going to be a skilled survivalist. You're not looking at a society that holds purges, or anything. Children who roll up 10s and 11s for all their stats and end up in shitty character classes will still be raised as cherished sons and daughters, but they're only going to make it to early adolescence, when it becomes necessary for them to pick up a sword and relinquish society's protection for the sake of their younger siblings. Those kids will be killed. Meanwhile, the members of their cohort who survive will continue to gain levels and eventually be the only ones replacing adult casualties and surviving to breeding age.
Obviously they'll still be a post-agrarian society. They'll seek shelter in fortresses to multiply their survival power, just like agrarians settled on farms to multiply their feeding power. And, as the agrarians provided small numbers of specialized fighting men to furnish their protection, the fortress-people will have a small host of domestic magicians who can feed everyone, leaving the greater fraction of manpower to organize a defense against the limitless hoard of the undead. But does that make for a society with skilled scientists or engineers, like ours does? Well, the thing is, technology doesn't really flourish unless you have a vast pool of idle and marginal academic talent loafing around, unoccupied with providing for their own existential needs. You can congratulate your Randian ubermensch for their brilliance as much as you like, but the intellectual establishment at large doesn't get any good forward momentum unless it has lots of marginal teachers who aren't up to the task of providing any meaningful breakthroughs who can cycle lots and lots of new students up to the level where
they can take a look at the outstanding inquiries of the day, with the hopes that some small fraction of them might have the talents and affinities to make breakthroughs themselves. Otherwise, you're just at a standstill, with all of your designated academic are set mostly to the task of learning everything that society already knew so that they can, in turn, teach it to the tiny handful of brilliant youths who will be their replacements.
Now, wizardry can certainly advance under those circumstances. You can make a new level 1 wizard via an apprenticeship and then give him every single spell in the hands of the academic establishment by having him crib notes out of his teacher's book. You can put him to work casting
Color Spray for a squad of fighters until he levels up and instantly gets to pick two
new spells out of fucking nowhere. You can have him, with no prior experimentation, begin labor on new wondrous items and architecture that are complete one-offs, founded on nothing that that particular wizard has ever seen or heard of before except by having a copy of the prerequisite spells. He can do all this because magical science is founded on total horseshit, not an ever-increasingly sophisticated corpus of knowledge and theory that grows only in diligent focus and which carries so much organizational overhead.
So, in this, one of the ideas you offered deserves special rebuttal; you suggested that when "all the resources are still there and split with far, far fewer people than before," its means that "the rare few scientists and engineers left [...] have far more resources at their disposal." And. I mean. I don't want to make to fine of a point giving you shit for this, but it's kind of an act of will, because this notion is
baffling. A scientist isn't going to have more resources after a massive depopulation event. A scientist's main resource
is the population. There is absolutely jack shit that he can do with fallow croplands or huge stockpiles of unprocessed iron ore; what he
needs is a society that, itself, has those things, so that they can keep him and about a thousand of his science-buddies all well fed and well equipped. What could possibly be left, after the wholesale destruction of the bottom 99% of the intelligentsia, that the remaining 1% could want? Less competition for government research grants? There wouldn't be a fucking
government.
But the base idea is still very good. There are three directions it goes in, depending on the conceits you accept when you put the world together.
Totally Annihilated Kingdoms. Okay so let's say that a ruinous depopulation event actually
does leave behind a bunch of important resources that are directly exploitable without a sprawling societal infrastructure. In this world, nobody's capable of doing any interesting magic without fabricating a lodestone and dropping it on a sacred leyline. But a single leyline can only provide the first spell level to any particular individual, and there are diminishing returns for them on every subsequent leyline. Also, every leyline is geometrically limited to only supporting a certain number of individuals.
In the old days, the greater potential of these leylines was debased by allocation schemes that favored broad domains and appeasement of a conditionally-passive aristocracy. So, when the plague came, there were comparatively few consolidated power bases; those that were are the only ones left. Now, the undead hordes are still out there, suppressing the ambitions of smaller men, but if one of the established powers was cunning and motivated, they could probably run the table... So that just leaves one question: Who wants to be a god?
Renaissance. The plague made monsters out of everyone who wasn't strong enough to resist, and then, when there were enough monsters, there
wasn't anyone strong enough to resist. Not except for some places, where the walls were high, the doors were thick, and the population consisted of an unusually high percentage of badasses. Many years later, these few remaining enclaves of civilization are still standing, most of them stronger than ever before. But here's the thing: they're at their limit. The shift to total war is complete. The population is doing everything they can possibly do. What's left to do but fail?
This is the situation: If these strongholds don't fall to the siege, then they will most certainly fail in their isolation. The survivor societies have stable supplies and populations, but they are experiencing attrition in the form of lost skills. In this setting, nobody is permitted to take the first level in a class except when they have received training from someone who already has a level in that class, and nobody is allowed to cast a spell which their teachers did not personally know. And in that environment, it's easy for the survivor societies to detect when a skill is lost irrevocably. They know that there are were people from before the plague that were educated in marital schools unknown. They know that when the chief successor of a magical tradition dies without attaining the mastery of his predecessor, that those spells which were within his potential become lost forever. And that's a problem because it's really difficult to take skilled manpower off the line long enough to do the research from scratch.
So here's the plan: They gotta hook up with the other enclaves. They know there are other survivors out there; every once in a blue moon, circumstances and luck permit a runner to slip past the enemy and share news with distant establishments. And it's easy to see that their strengths could be one's own strength, if only there was a way to combine arms. So that's gotta happen. Somebody's gotta dare to break the impasse, to swing their gates and issue an offensive action. They've gotta open up one of the old roads and secure a line with another enclave. And they need to maintain their momentum and do it again. They need to build a triangle, patrol its exterior, and sweep its heartland for the ruins of civilization and bring back something that nobody even knew they were missing. And they've got to have to conviction to keep it up, as well as the prudence not to overextend. So... Who's gonna do it?
Points of Light. So let's say you're an uninspired twat and want to pretend to have achievements without actually moving the status quo. Your players, being self-obsessed and interested only in devising further angst for their characters' backstories, are amenable to this state of affairs. So you're going to come up with a name for an ancient empire (or possibly more, if you'd like to give a couple of your core races their own in a failing attempt to make them interesting) and pretty much nothing else, because they're all dead now. It was the hubris of man. Not the kind that contributes to a complex series of failings which results in the slow decline in influence, though, because you are, in all likelyhood, a sociological moron. Nah, it was just some magical cataclysm that blew 'em all away and left a bunch of sedentary boss monsters in their basements. A thousand years later, your PCs kill the boss monster and find a +2 sword. Then they go back to a little town that was simple enough for you to conceptualize during that study hall when Jill was absent and you needed something to daydream about that wasn't a contrived opportunity to fuck her. In that town, they will hear about another basement with a different boss and go kill him, too. And at some point Jill might be absent again, so they'll get to go to a different town and do the same kinda shit over there. It's all good.
