I agree. Chasm attacks should be put back in the game. After interacting with the chasm enough, packs of whatever-men (or some unholy abomination, such as a gremlin) will leap from the chasm to attack your fortress. The groups of enemies just hanging around the chasm don't cut it for challenge, and once they're gone, they're gone.
As for the cave river, attacks there would be nice too, but I think it could be handled differently. A group of aquatic enemies could travel through the river every once in a while - at least twice in a year, more if you interact with the cave river frequently - with enemies breaking off from the pack as they find things that interest them. They'd start at either end of the river and move toward the opposite end.
Magma attacks are tough to deal with, and settling on a volcano isn't easy with fire imps and the two varieties of fire elementals hanging around in some limited number from day-one, but once you kill them off they don't come back. Suddenly that elaborate anti-magma-creature defense you had to erect because of the Fire Men that were threatening to burn your fortress down is totally worthless, because the magma creatures don't return. I'd like to see something similar in principle to the river creature packs done here. It'd be neat to have groups of magma enemies appear at the bottom level of the map and proceed upward through the magma vent before finding a place they can exit at to attack your fortress. Not only would this maintain the challenge of settling in an area with a magma vent, you'd also be ensured a steady supply of fire imp bones, provided that you survive the attacks.
You know what else would be nice? Something that got mentioned here, ocean attack groups. Maybe packs of flying hellions of some kind too, or pterodactyls or something. That'd be awesome.