A while back, I was playing a bugged version of DF where waterfalls didn't work right -- water would pile up at the bottom, causing a huge flood. I'd usually play in levels with a canyon and a river at the bottom, so the canyon would rapidly flood, reaching the end and pouring out everywhere. This was a pain, but it actually made for a fun challenge -- could I dam the flood before it hit?
The challenge was fun enough that I'd like to see it worked into the main game somehow, though obviously the player would need some warning. Floods have been mentioned as ideas before, I know (there was a sort-of flood in the 2D version), but I think it's worth mentioning how interesting the challenge of rush-building a dam can be.
Additionally, and this is what had me thinking back to it now (and why I posted it even though I know the first part's come up before): There could be dams in worldgen, similar to the new bridges. They'd make for interesting structures, and could eventually be used in adventures and plots (eg. 'the evil warlord X wants to destroy this dam, flooding the lowlands!' Etc, etc. Failure -- or it happening elsewhere, as a random event, or the dam failing from natural causes -- could be an interesting event that would affect many nearby things, including possibly player fortresses in Dwarf Mode. Or the player could smash the dam themselves...)