Magical change, you say? I would say giving DF real-world level graphics, but... that would actually be really awesome. Why wouldn't I say that? In fact, make it 100% immersive! I want to be Manba Spokenward the Geared Noose of Lashes, dammit!
Strangely I wouldn't. I actually would be against any (mandatory) graphical change that goes beyond ASCII. One of the huge strengths of ASCII art in games is that by virtue of its extremely minimalist style you have to fill in the blanks with your own imagination. I can let my mind create what my fortress looks like, what an individual dwarf looks like, what weapons look like, what an epic battle looks like, etc, using the help of the descriptive tools that already exist. I can fill in the blanks with my mind and see the haggard dwarf pulling a bucket of water out of a well with one hand cause his other was torn off during an epic battle with a goblin, his chipped bronze war hammer slung over his shoulder and his tattered armor hanging in pieces on his body. Even if we had graphics that were indistinguishable from reality, literally as perfect as they could possibly get, I would lose this. The dwarf would no longer look like I envision him, it would look like whatever the graphical coding said he looks like.
If I could magically change/add things to dwarf fortress it would probably be the removal of FPS issues (multi-core, whatever) and/or the addition of features that allow sieging enemies to break down/dig through walls. I would love to have a full on battle with the enemy where I could use everything I had to defend and they used everything they had to attack.. But right now that cannot be done.. Or rather it can be but I have an almost certain victory if I do so because of how overpowered walls are, I can just turtle up and pepper the enemy with bolts.
Another thing I'd love to change would be better working siege-style weapons like catapults, ballista, maybe even cannons*. It would be awesome to have some more massive battles in this way. In fact a simple way to help it work better now would be to be able to assign dwarves military style to the machines. The main flaw is that when the enemy nears they run away, which makes sense, but often results in them not firing the weapons.
*I know, no 'gunpoweder', but while I wouldn't want to see firearms in DF, I do like the idea of high explosives in the form of cannonfire or even in traps.