Here are some of my ideas of the kind of moves the Dragon King should have:

Fire Breath - The Dragon King spits fireballs, or a long lasting flame.

Acid Spit - He's half Reptile, so I think he should be spitting some sort of acidic projectile at people (something like Kintaro's fireball spit).

Shoulder dash - Reptile had the slide, but I think the Dragon King would be too massive too be able to slide under opponents (but you never know)! This move should be something like Shao Kahn's shoulder slam move.

Tail sweep - something like Motaro's tail sweep, tripping the opponent down and leaving them vulnerable.

Invisibility - My personal favourite! Like Reptile, he should be able to make himself invisible at any time! I think it would be cool to fight a boss that can turn invisible!

Dragon stomp - Something like the move Hsu Hao could do where he jumps high into the air and crashes onto his opponents. I think the Dragon King will have wings, but doubt he'll be able to fly (due to his size, but again you never know), so his wings allows him to leap really high into the air, and come crashing down on the opponent. Alternatively, he could have a stomp move like Goro / Sheeva.

What kind of moves do YOU think the Dragon King ought to have?