Have you played Dungeons and Dragons? It's not supposed to be fun as a place to really live, living in a land harassed by monsters, demons, no security, etc. It's an interesting scenario to explore creativity and conflict in difficult situations.

Sure, but imagine if a D&D setting was an actual world that somehow survived long enough to start implementing countermeasures (remember the last 1/3rd of this book is set thousands of years in the future, so reasonably analogous). If the Player Characters decide they don't want to bother with a problem, it would have to be solved with other methods, like massive use of defoliants (think agent orange) and pesticides, bunker buster bombs (think MOP, if deliverable with enough precision), and as a last resort, dropping massive thermobaric weapons to destroy entire cities, possibly repeatedly, and then setting up miles wide demilitarized zones in a circle around the incident zone with kill-on-sight orders and automated turrets and tracking systems, if feasible with the tech available. Obviously a few Player Characters can't handle everything, so it would be reasonable common as an ordinary person to find out the next city over no longer exists and they have just been drafted to man the "human side" of the newly created DMZ.

You're right, I had assumed some kind of general peace for the resettlement of the surface of the planet. Given that, there would be some lawlessness, but at least no orbital strikes/nukes.

I was more thinking about various types of agents from the people living in space, all infiltrating each other, traitors to their own types, double agents, weird combinations etc. Plus mysterious people nobody can place - who turn out to be agents from the submarine/hot air balloon groups. And vice versa.

The land is now mostly open and clear, right? So there will be new populations, settlements being started down on the surface. A resource race, with possibly high-tech stuff lying around, relics from the lost civilization!

Player characters could start off as an alleged group of "explorers for sites for new cities" put together by a newly established base - but actually one is trying to divert them away from the hidden holy site of the mineshaft people, where he came from, the other is a spy from a lost faction of the brain people who are outlawed in space now, another is trying to fulfill a 5000 year family promise he received from his father, that when he could, to go to a certain place and say a certain prayer, etc.

There is just huge diversity of backgrounds and tech, and a large, unexplored environment potentially full of strange treasures!