Bukkit

Random explosions:
Exept for all the fireballs, there could be completely radom explosions. They could be coded to occur mostly near blocks and items that indicate structure and/or player activity, such as planks, glass, cobblestone, workbenches, item drops, large concentrations of farmable mobs e.t.c. Could also be tied to hostile and passive mobs, making most mobs potential creepers.

Mob floods:
Already included, just be sure to use plenty of ghasts, blazes, cave spiders, withers, and why an Enderdragon or two?

Configurable floods:
Most apocalypse elements seem to be fire-based. I think there should be a water-based apocalypse and a fire-based apocalypse, so that they don't limit each other.

Dry Oceans:
Why not remove all water? That would be pretty apocalyptic. (Is that even a word?)

Hurry up, I think a lot of players will be quite intrested in this in about three days.