So, what got me thinking about this is that Dragons (as well as Giants, and other mega/semi-mega beasts) don't seem to do much in Adventure Mode. Sure, you can go hunt them down in their caves and fight them, and all the poor, helpless villagers will cower in fear of a dragon located halfway across the world. But ultimately (in Adventure Mode, at least), all these beasts ever do is stand around and sleep inside their lairs for years on end, waiting for an adventurer to kill them.
Why not allow the player to take control of a (semi)megabeast, in a new game mode? Allow the player to scroll through a list of available monsters and beasts that exist in the world, and choose one to play as in a new, Adventure Mode-like game.
For example, a player could control a Dragon, who lives in a lair/cave. They could hibernate to pass the time, waiting for adventurers to show up to attempt killing them. They can go outside to hunt, attack villages & forts. They could gradually build up a hoard at their lair. The ultimate goal could be to survive as long as possible (since megabeasts are very long-lived). Other activites, such as expanding their lair, finding a mate, fighting for control of territory with other megabeasts, etc could be implemented. There would also be differences depending on what beast you are playing as (i.e. dragons breathe fire, rocs fly, bronze colossuses don't need to eat, etc.).
Of course, this idea could use a lot of fleshing out, and I'm not entirely sure how difficult it would be to implement. But I, for one, would love to be able to play as a (semi)megabeast, rather than being limited to civilized races.