Some Questions And Ideas On Mobs/bosses

While there are plenty of different mobs for flavor, most of them act the same. There are some unique minibosses (like the Sun God) and bosses, but I've noticed that most boss fights quickly get repetitive. For this I have questions and ideas.
To start off with the questions, what do GMs, Builders, or whoever creates the mobs and bosses have to work with when they are creating one? Is there an interface? How are special abilities added? Can I have an explanation so I can understand why most of the mobs/bosses are like they are?
Now, for the ideas.
-Most of the mobs have standard Minecraft AI, sometimes modified to fit in a special ability. I know that recently flanking mob AI has been added, but I have a few ideas to do that.
For example, could a fight-bot AI be tweaked a bit and then put into a boss?
A fight-bot is a kind of hack that flawlessly PVPs for the player. Could this be implemented into a mob?
-Another thing I've noticed is that a lot of bosses have special abilities that copy a single player spell. Why not more than one spell? Why not all the spells in the class?
-Most mobs I've seen have one of 2 basic attacks: a melee hit or an arrow-like projectile, with a few other attacks from the likes of blazes. Why not mage-like attacks? Why not grabs? The special abilities can really make a boss fight, but an interesting mob or boss could come from good default attacks.

 

They're created with custom plugins, programming, and Command blocks. Nothing simple like a UI.

 

Sorry.
But still, could a fight-bot be implemented into a mob?