So i was just fantasizing about what we could do in DF in the far future when Toady implements a lot more stuff, like becoming a pirate captain as an adventurer and burying stolen loot on deserted islands and making treasure maps to hide in player made dungeons.
Then i realized, we can't bury things yet.
So i suggest that we get the option to fill in spaces with dirt, and also cover stone surfaces to allow farming if water is scarce. The dirt would be collected like sand is. Basically allow dirt 'floors' and 'walls' that just act like unmined dirt instead of constructions.
The 'walls' should only be 'buildable' from the level above so we can fill in pits and make dirt walls harder to make (since dirt is dirt cheap). Perhaps even dirt ramps/staircases could be 'buildable' as well. It should act like formed obsidian or ice to 'contain' items and creatures. Yes this means you could bury goblins and nobles alive. It should take 3 whole bags full of the same dirt to make a wall or floor though so it won't be too easy to make them.
Another application of the bags of dirt would be to disguise floor hatches as dirt floor. Collecting dirt would also open the way for clay being used to make ceramics (Yay, more use for the magma kiln!).
Also you could put the dirt in a jar and gloat about it. Sand should still be usable since they're not just pouring the dirt on, they're packing it in too.