daniel_gudman said:
So let me step back and ask more generally:
What should i be trying to accomplish story-wise with "the disaster", not just the mechanical effect of removing [Safe Zones], but what the impact on the thoughts of the people inside be; and what does that imply about what the nature of the disaster even is?
As I recall, the purpose of removing [Safe Zones] was twofold: One, to force players to invent bounded fields to recreate them. This was the reason to remove safe zones on future floors. Two, to force players who had remained inactive to participate somehow. This is the ultimate purpose of various disasters proposed.
I don't think the disaster should be triggered by players unraveling a bounded field or anything. You don't want to teach the players that [Progress is Bad]. Players should never be punished for advancing magecraft, so it should be an action of the boss.
On the subject of the disaster, perhaps the boss releases some sort of corrosion effect? Something that slowly disintegrates objects, starting on the first floor and working it's way up? First, buildings fall apart, then objects, then NPCs, then finally players start taking damage just for being there. The effect can be blocked with magic resistance or bounded fields, but the effect constantly gets stronger. So players are forced to either migrate to a higher floor, or develop increasingly advanced magecraft to stay there. Either way, Kayaba wins. The [Magic Teacher] NPCs can be the only ones unaffected, which can teach players how to beat the effect. This also allows him to provide a pressure from below, to goad otherwise inactive players. If the corrosion is constantly advancing, spreading between levels, players will realize that they NEED to clear more floors, just to protect themselves, even if they aren't front liners. This gives everybody a stake in the war. Also, Kayaba can make it explicit that it advances erratically, faster sometimes, slower others. This allows him to secretly "tune" the difficulty, making it slower if things are going unexpectedly badly, but turning up the heat later to compensate. Since the players don't know that it's rigged, it seems equally threatening all the time.
Really, making it explicitly an action of the boss still risks people saying "we would have been better off if we hadn't advanced" so maybe make it something that looks like the players have a chance to stop the boss from doing, but they actually can't? Well, maybe that part will just have to be suboptimal.
The other caveat to destroying the lower floors is, there will still have to be some mechanism whereby low level players can practically level up. So either have some sort of "learner" area on upper floors, or destroy the lower cities and safe zones, but leave the portals and floor monsters otherwise intact so that players can "commute" there to level. Players don't just need "game experience" but also "real experience". Still, this may be another area where players just have to live with things being harder, and work with higher level players.