This would be a very interesting thing to implement, craters, filled with water or not, would be an interesting feature, especially if they contained small, not quite epic, materials and meteor fragments. There could be some unique animals, or have it returned to normal/act like a desert badlands.
If they're more than geological and can fall during world gen, the same could be true, but it would leave a blight of desert of a certain size But not affect the climate beyond that, else it would be hell to generate, especially from a parameter set. Increased volcanism in the region struck would be nice, a few small magma pipes and pools in the region. It could destroy any nearby cities and towns as well, creating ruins farther out and destroying all nearby, Dwarven halls may be exempt from these effects.
If it is possible to have happen during game play, outside the region the player is currently viewing, there could be a heat wave and dust storm across the map, acting like a wall of infinite miasma and leeching into structures, extending a distance relative to the meteor's size, even across the world, which would kill everything caught on the surface (outside, light) and raise the temperature up to the level of magma. This would be possible to ride out underground. Less destruction from smaller/more distant meteors.
A large impact on the same map as the player's fort/adventurer would potentially destroy everything above a certain Z level, and leave a large crater around the point of impact. the smallest impacts may leave only a small spot of obsidian down a z-level or two and start a wildfire. Heat could also be relative to size and distance.
As per the metals, I'd say allow for a stronger version of iron and nickel, Mithril from the dig deeper mod in the core, names are meh.
For creatures, HFS could arrive after the impact wave, or mutilated/undead versions of normal wildlife, from many biomes downstream, in a swarm from the same edge as the wave, maybe more than once, maybe not right away either, it could take them a season or two, and they could act like usual enemies and siege you. Zombie siege... Allowing this plague to occur in a handful of your own survivors would also be interesting. They could hang around in the craters for centuries if during world gen. And spawn from those that land at the doors to your fortress, in swarms, maybe with quite a few just leaving entirely. And let them wield precious and powerful weapons! If there are any special creatures of course.
Fit in procedural generation wherever possible, just for the hell of it if that suits your thoughts.