I don't think anyone's mentioned glacier deposits.
I'd like to see masses of jumbled stone being pushed forward incrementally, in front of glaciers, and then "devil's garden" type debris areas, where glaciers have melted.
Also, how about (regular sized) termite/ant/gopher/etc "cities"? Places where normal animals have, themselves, significantly altered the landscape?
How about archaeological sites/structures,
and sites where intelligent beings have left paintings/graffiti?
Such as the one at Tsodilo (contains over 4500 rock paintings, and is the site of the oldest known ritual--70,000 years ago).
http://en.wikipedia.org/wiki/TsodiloIt'd be cool if our dwarfs occasionally find flint arrowheads, old statues, bronze coins, etc. buried slightly in the soil. Maybe even an amphorae filled with ancient wine. These could be turned up by our farmers.
Speaking of lava, and farmers, are there different types of soil in the game? How about lava transforming into rich soil over time?
Occasionally having huge bodies of fresh water (great lakes)--as opposed to salt water--might be interesting. Also, areas of water with a high level of salinity (great salt lake, dead sea, etc.), and small pools of alkaline (etc.) poison.
How about caravans that travel with a significant population? Like a sort of nomad/gypsy town, or just a huge caravan-train. Might arrive with a herd of pack-animals (camels, dogs, other).