standard suggestion boilerplate:
OK, well, one of my favorite elements of dungeon keeper was the libraries. For those of you who have never played the game, you could designate rooms as libraries, assign warlocks to them, and watch them research all kinds of goodies.
In the case of DF, I would love it if we could make libraries in our fortresses. In fact, I think every fortress would have to have at least a basic library to function beyond a few years in a friendly climate. I'm not suggesting a room which functions as a 'tech research' per se, but rather a repository for knowledge.
Lets consider world gen. Currently we have races, civilisations, mega-beasts, heroes, artifacts, all dynamically interacting over a massive span of time. I would like to see Toady add a new, special kind of unique artifact object into world gen = tomes.
Imagine that a tome is like a blue-print. A lot of basic knowledge any dwarf worth his salt would simply know, even if they are not so skilled in the actual technique. Take barrel making, for example. This could be counted as 'common knowledge'. However, what if certain types of construction that are suitably complex/cool required access to a tome. You set a nice fancy trap to be built, or a high quality bridge, and your architect runs off to the library to consult the relevant book?
During world-gen, dorf civs could create these tome objects, which could spawn into copies as they circulate amongst the different fortresses in the world. When you start your fortress you could have a few books with you, but as you develop you would want to start trading for them.
This whole thing could open up new occupations (scribe, librarian, etc.), new material chains (parchment, ink, etc.), and form a basis on which to build other interesting additions.