With #2 I didn't mean a new chasm appeared, I meant monsters from existing chasm biomes and lakes and such appeared. That would be a great one to have implementable in game: some kind of trigger that causes an inexorable assault out of the chasm/river/magma pipe.
The others, well: it would be tons of fun to have more ways to overrun the fort, but I think some people are kinda missing the point. It's not to make playing the fort at the time more fun, it's to link the fortress up better to adventure mode, or to reclaim, to make exploring fortresses in those modes more interesting. Like I said before, having a monster invasion would be awesome an' all, but it wouldn't be at all what I'm describing. You can't set that up for an adventurer, you just wind up with a messy fortress full of randomly dispersed monsters. You can already do that: open up the pits, let a goblin siege take over, etc.
Anyway. Having engravings or scattered scrolls and books or just engraved items spawn to describe what happened, especially for #1, would be awesome. Then even the player of the old fort would have some surprises. It'd be sort of as if the player left, and the horrible events transpired after that. Making the stories randomly generate in a coherent way might be a bit tougher, but that does seem to be toady's forte.
"An image is drawn on the wall in Dwarf blood. The image is of a dwarf and a zombie. The dwarf is striking down the zombie. An image is drawn on the wall in dwarf blood. The image is of a dwarf surrounded by zombies. The zombies are groaning. The dwarf is making a plaintive gesture."