With the massive success of all the Iron Man films, and Robert Downey Jr open to doing another solo film, it makes sense to make the fourth Iron Man. Perhaps Iron Man 4 would be made in Phase 4.
If Iron Man 4 is going to happen, then it would make sense to use up characters fans have been craving for. Of course to many villains may case problems, perhaps to solve this Marvel could have Iron Man and War-Machine team up with the Hulk, or other Avengers members.
Do you remember in 2010's Iron Man 2, how Tony Stark said it would take people 5-10 years to develop the suits, well the 5 years are up and it is about time we see more suits. But which villains should appear in the film, well here are some predictions:
The "Real" Mandarin
The Mandarin is the leader of the Ten Rings, a terrorist organization that kidnapped Tony Stark, which led him to become Iron Man.
He was previously thought to be a fictional character created by Aldrich Killian to cover up the Extremis attacks, but upon Trevor Slattery's kidnapping from prison by an actual Ten Rings agent, the truth of the Mandarin's existence has become all too well known to everyone involved in his schemes.
I think we can all agree that the reveal of Trevor Slattery's "Mandarin was pretty much one of the biggest let downs in Marvel history. At least they added the ending they did in All Hail the King, revealing that the Mandarin actually exists. It may happen or not, but at least he is out there in the MCU, and if he does pop up he will be one of Iron Man's greatest enemies.
In the comics Anton Vanko was the original Crimson Dynamo, but seeing as he died in Iron Man 2 as a old man, it is highly unlikely we will see him reprise the role. Then we got a look at his son Ivan Vanko who took up the mantle Whiplash which overall was a good performance, but did not last too long to unveil the Crimson Dynamo.
Perhaps in adaption for the MCU, Marvel will use one of the Crimson Dynamo's other ego's, like just off the top of my head Valentin Shatalov. Valentin Shatalov would be a high level agent of the KGB, and seeing how the world is changing with people like Iron Man, would do what is necessary to bring Russia back into power. He would then dig up Anton Vanko's old research (as a nod to the original Crimson Dynamo), and use it with modern technology to develop a new wave of Iron suits to restore the Soviet Union. Because the Crimson Dynamo is Russian, this could led to a team up with Titanium Man, or maybe the two could even go so far enough to join the Masters of Evil.
In the comics Boris Bullski aka Titanium Man was a highly skilled agent of the KGB and eventually was partnered with the Black Widow. When Tony Stark's bodyguard Iron Man was shown to the public, the Soviet Union decided to show their superiority to the United States by creating a powered armor of their own. Made from titanium, a stronger substance than iron, the armor was larger than Iron Man's yet technologically inferior. Bullski used the armor to challenge Iron Man in a one-sided fight, though Stark managed to win.
In the MCU, Marvel can make a similar approach to the character by making him a KGB agent volunteer for the suit. As for how the make the suit, maybe the restart Anton Vanko's research and get to work on designing a suit that can rival Iron Man. When the suit is completed Boris would inherit it, go by the alias Titanium Man, and go on to fight Iron Man. This character could be a great addition in the MCU, giving Iron Man a equal level foe, and possibly making him a member of the Masters of Evil.
Donnie Gill has already been introduced into the MCU, though not exactly as the same level as his comic book counterpart. He has made his only appearance in two episodes of Agents of SHIELD, where he was a SHIELD student who worked for a Ian Quinn a corrupt businessman in designing a Weather Machine. The result wound up being Donnie being struck by the machine and placed under SHIELD observation. Afterwards it was discovered that the shock from the device gave Donnie the ability to freeze upon contact. He then went rogue and before he could be claimed by either SHIELD or HYDRA, Daisy Johnson shot him and watched as his body sank to the bottom of the water in a ice cocoon.
But Daisy mentioned that Donnie's body was never found, which could be an indication that he can return. If he does then he will no doubt have a vendetta against Daisy for shooting him, and might even go as far as to rejoin HYDRA of his own free will, in order to get revenge on SHIELD. Perhaps in order to augment his powers and give him protection from bullets in the future, he could be equipped with a specially designed suit meant to enhance his powers. Then he would truly become Blizzard, maybe run into Iron Man, or better yet join the Masters of Evil.
In the comics, Arthur Parks was a brilliant scientist in the field of laser research and design. He became a career criminal using laser technology, and then a being of pure living energy.
In adaption to the MCU perhaps Marvel could have Arthur Parks take a similar route, maybe even have him as a AIM scientist that would experiment on himself to gain power, along with a suit of armor that would allow him to control said power. These experiments would then lead him to become the Living Laser.