Despite the title, all these suggestions are carefully thought out to be eminently survivable with a bit of forward planning but still have the potential to tax your crisis-response skills.
Earthquakes: Every wall and constructed floor in the fortress has to pass a defence roll that varies in size according to the size of the quake. Some regions should get more of these than others; I suppose Toady could even model actual tectonic plates if he wanted to, though how that'd mesh with the HFS I've no idea. Frequent earthquakes should probably be something you're warned about on embark a la acquifiers.
Forest fires: I think these might actually have arrived with the overhauled temperature system, but dead grass and shrubs should have a much lower flashpoint. In the interests of continued player sanity it might be a good idea to hold off on this until bucket brigades are implemented.
Landslides: This would be a real bugger to implement, but lots of Fun and easy to weaponise. The game would have to track how wet an earth tile was becoming (
edit: turns out this is going in anyway, as part of the irrigation overhaul), and if it reached a certain point of saturation it would turn into "mud", which behaves like cold magma that sets rather than evaporates. If it happens to enough tiles in one place, it changes the terrain, not necessarily in ways that the player is happy with. A tree on or adjacent to a tile should protect it from turning into mud.
This mechanic would also open up the possibility of quicksand.
Floods: Could be seasonal, based on regional rainfall or a bit of both. Brooks turn into minor rivers and thus unwadeable for a while, larger rivers slowly overflow their banks and try to fill up the next z-level. Some floods could happen slowly enough to build walls to contain them, possibly out of sandbags, but occasionally you could have flash floods capable of demolishing bridges and sweeping away anyone unlucky enough to be in their path.