The probable reason for this however is that Toady One wants to see megabeasts in the game but does not have an AI sufficient to have to behave in the calculating fashion they are supposed to.
But?
Aren't siege creatures running away once injured? Entire groups of recruits tend to experience mortal fear once they see their capitans being pulped by elephant men and run away shortly after.
I think that AI is here already - just for some reason Megabeasts and the like are not using it.
I basically dropped that giantess on a pile of very expensive items and a path to the edge of the map open.
Instead of stealing anything she got pwned by that kitty on the chain.
Probably we need to define their motivations in order to accomplish anything. The megabeasts need to make an estimate of the threat the player's fortress forces pose to them and then give them a specific objective to complete. We compare their motivation level against their threat level to determine when they flee, completing their objective greatly reduces their motivation so the threat level matters more. Taking serious injuries greatly increases their threat level while slaying fortress soldiers (not random civilians) reduces threat level potentially to nothing or below, so that the megabeast will simply kill everybody until it owns the whole fortress.
That would be awesome.
I would add to that, once MB is successful for the first time, it would be more likely to come back later as they do in the legends.
This creates a whole set of dilemmas, it may be better to not fight the megabeast and let it just take what it wants under some circumstances. That is because they tend to think that the soldiers they face in combat are the best soldiers you have, if they trounce them without injury they see no reason to fear whatever else you might have to offer and so can decide to take everything there is. However in the case of [INTELLIGENT] megabeasts it might make sense to engage in diplomacy with it, allowing it to take what it wants without a fight while if were have a megabeast that has trounced you, it might make sense to offer it citizenship and a high-level position in return for it not killing everybody.
Basically create a stockpile of prepared food outside hoping that the beast will eat roasts instead of making roasts of your citizens.
Or make it an object of worship for your fort, and in exchange it takes residence and your dwarves constantly bring it food and drinks to keep it pleased, or it eats them otherwise?
Hell, it would make a greater difference to the gameplay than locations and new tree system.
Suddenly the dragon could became a god of the dwarves and who knows? Maybe let them forge some legendary items with help of it's dragonfire?
10/10, would gladly make offerings from a captured siege races to satiate my new resident object of worship hunger.
Somehow a scene from Kohan 2 comes to my mind:
Drauga Warrior: - foolish Ceyah! We've come here to worship our god!
Drauga Warrior: - Sakkar is strong! How fares Ahriman?
Melchior: - You can ask him yourself! (kills Drauga)