not sure weather it's been mentioned or not, but salt water under ground rivers do exist in RL, it's just that they aren't very big. but this could be a rare feature in DF.
also, most caves are only 1 or two tiles wide. huge natual caves 5 or even 12 tiles wide, going many more tiles deep would be a nice feature.
also, how about several z levels below you, a hidden goblin civilization, trapped there for many (world) generations, ever since a cave in, has finally breached to the surface, and due to, say, LOS, although you can see some of the underground cave that has just opened up and erupted with golblins, you can't see all of the new area until you send in a dwarf to explore, in which case you find the goblins have invented 23rd century-type technology, and a space-ship wing takes off the dwarfs head.
translation: lost civilizations could invent technology way beyond your own, depending how old the world is,but not be able to show the world until they are freed from thier underground prison(s).
a page back, i read about how certain features dissapear fast. i can agree. ever find a tropical glacier? turns out it was a flying wagon 7 z levels off the ground, and 15/7 water, with not a trace of ice.