4. Fin Fang Foom
I'm not a huge fan of the character in the comics, but an on-screen version could end up looking fantastic if it's done right. The Iron Man movies have been a great success, and number 3 looks set to exceed that of those before it, but never has Stark had to go up against such a monstrous foe on his own. The sluggish Leviathans he faced alongside his Avengers team-mates were big and caused a lot of collateral damage, but they didn't have much else going for them. Foom however, who was first introduced in 1961, is a giant intellectual dragon with regenerative abilities and the ability to fire a combustible acid mist from his mouth and would be a different prospect altogether. And seeing Foom going up against Stark on screen would surely be a sight to behold. So, mainly for the potentially outstanding visuals, Fin Fang Foom would be a great addition to the MCU.