When you start to make a game, it's always wise to do it in some way, capable of going here and there.
Please. From the very start, make you geometry database very flexible, perhaps including lots of small files with every particular game zones. This will allow you to change the world rapidly and with very little downloading for your clients (us).
At some point after release, it could be brilliant to set up some of developers team to be a "progress-party", who will be thinking about new ways of world progress, making those changes in-game all the way. Let them not stop. New villages, discarding old ones, relocating raiders camp and their (and not only their!) patrolling routines, new trees growing, floods, earthquakes, new respawn nodes and all and all. Not I say those changes must be done for some "lore purpose", but to make the world always new even for a very old player, who - he thinking - know everything, but - bah! it's a cruel word, you can't mess with post-apocalypse! - you are "defeated".
Please. Please, please, please. Do not make such a game as another game for pussies. Must be a blood, gore, and cruelty - players must live in cruel world. 16+ would be perfect, FO1-2 fans grows older. (Heh, you thinking 12-y.o. will not play it? Yes they will anyway!)
I'd recommend you not to set real hard spawnpoints, like in Wow. I was always thinking if I was a game designer, I'd invent a system of dynamic spawn zones. Imagine the family of rabbits (Spawnzone1). There must be few (not 2, but 5-6, may be, depends on what point spawns type) "parents" (breed? don't know English too good as you could see), who will be locked to the, say, center of spawnzone, roaming here and there around it. The zone will spawn those rabbits, who will roam to any direction, free to go anywhere and make people and mobs more happy, when meated (huimor joke). If not enough parents, some new spawned rabbits can take it's place. So, they all spawn in some location, spawned rabbits hip and hop here and there, passing layers shoot them dead here and there for food and for profit, idyllic! Spawnzone of any type can be spawned on very particular ground, like rabbit spawn zone doesn't ever spawn or move to the swamp, river or mutants lair. Only grass (meh), forests or jungle. If the zone attacked twice from, say, south, it moves north a little, so zones moves like that. Imagine the family of volves (Spawnzone2). It have the same class ( I mean class Spawnzone{some code};), with different attributes, defining be it mammal, mutant, resource, etc. Imagine Spawnzone2 start to overlap Spawnzone1. Spawnzone1 moves from Sz2, and so on. This process must add world much flexibility, not taking so much effort from developers. Also you can set up zone's experience level, allowing long live zones to produce mutated denizens. Imagine goddamn rabbits, who starting to hunt down and eat volwes instead, bleh.
if you can translate it from Runglish to English and read, perhaps you find ideas interesting. Discuss!

While I think constant re-development would make for a more living and engrossing world, I doubt it will happen due to the costs associated with it.

If coded right, and with professional approach to the process, cost might be reduced dramatically.