DF has really broken the rougelike mold in many ways. I recently had a cool idea for megabeasts, though, that would bend the RL genre even further.
If you think about it, its really quite anti-climatic when you discover a dragon for the first time, and visually he is the same size as you. I know that the "size" field is different in the raws, but what does that change? He's harder to damage and he deals more damage. That's it.
Now imagine: You set off on a quest to kill your first dragon. You come to the cave and venture in. After some series of tunnels you find an enormous, open cavern. Filling about half of it is a gigantic pile of gold, and sleeping upon the gold is a dragon -- half the size of the cavern. You are stunned beyond words -- how are you supposed to kill this brute?? Armor covers almost every inch of his body, except his underside. You can see his razor sharp teeth and sulfurous smoke bellowing from his nostrils. Then he wakes up.
Thats the end of the narrative. Now imagine actually playing the game at this point. All you can see are the dragon's front feet from your current z-level, but up 4 levels is the brute's head. A single adventurer would be almost completely helpless against a beast such as a dragon. The dragon would be able to smash the ground with his feet, sweep it with his tail, or just light you up with his breath.
But imagine commanding an army against such a fearsome foe . . . and winning. Your pikemen would charge the dragon, 4 or 5 getting flung away, but some would make it to the soft underbelly, and penetrate the dragon's insides. At this point he'd be weakened, and the vollley of arrows would pierce his wings so he couldn't fly away. Then you'd finish him off, gashing a mighty wound in his neck.
That'd be epic.