I haven't really tried out QQ yet, nor have i read much of the comments here, so sorry if these ideas have already been mentioned/ or are already implemented:
1. Random teleport - players within the specified segment are teleported to a random location within the segment OR 2 (or more, up to all) players within the segment would be teleported together, and possibly locked into a mini-deathmatch arena for the duration of the disaster. This could be used towards the end of the game to speed up the normal deathmatch. Various effects could be applied to the players as part of this mini-deathmatch. (e.g. jump boost, strength, weakness, slowness, invisibility, etc...)
2. Paralysis - players within the specified segment would be trapped, unable to move from the block that they are on for the duration of the disaster. (Or have slowness applied to them.) This would be a handicap to them as players in other segments can still continue to progress through the game at the same rate, and hence would acquire better gear faster. (Obviously dependent on what stage of the game this disaster occurs as to how much of a disadvantage it causes.)
3. Poison fog - like in Catching Fire - starting from the edge of the map and moving towards the centre. The animation could be done through either the TNT animation or void fog. If a player was caught in it, various potion effects could be applied to them (i.e. poison, nausea, hunger, instant damage, blindness, etc) OR they would be given a certain time (maybe 15 seconds) in which to get into water, (like in the movie) then, if they didn't make it to the water, would have the effect applied to them. (Or simply instant death.)
4. Simply have a Wither or 2 spawned in (maybe with some wither skeletons) - That's enough of a disaster in itself...
TL;DR - Bad stuff happens