5). Gorgon
Who is he? The agents of S.H.I.E.L.D. have fought against Daniel Whitehall, the Strucker family and Hive--all who appear in some fashion in the comic series Secret Warriors. Gorgon is another villain from that comic, and perhaps the most interesting out of all of them. In the comics, Gorgon is a genius mutant, but he doesn't have a mutant agenda, which I think would be a nice spin on the current inhuman storyline if they were to make him an inhuman. He turns people to stone by looking at them, and is a member of both the Hand and Hydra.
How should he be used? Hydra definitely needs a long rest, but I don't think that means they should never be used again. Gorgon is not quite as ancient as the show has made Hive, but he's still incredibly old. I would make Gorgon the first Black Sky who turned on the Hand, as we know from Daredevil that although they are rare, there is more than one. This would provide a link between the two shows.
One of the coolest aspects of the Secret Warriors comic was the fight between Gorgon and Phobos, and their respective swords Godkiller and Grasscutter (it'd be cool if one of these were Asgardian in nature, making way for another Lady Sif appearance). I'm not sure what his motivation should be besides power, but I'd make his followers traitors of both the Hand and Hydra, who decided to follow him instead of their respective leaders.
4). Phobos
Who is he? Phobos is the Olympian god of fear and son of Ares, who was an Avenger in the comics. Wikipedia said it best: "He possesses the power to instill fear in others, and has limited pre-cognition. As a member of the Olympian race born of a mortal woman, he has the potential for far greater ‘god-like’ powers not unlike his uncle Hercules, but his ‘mortal’ body must die first." He reminded me a bit of Damien Wayne, except Phobos is actually a good character.
How should he be used? I would make Phobos the "Scarlet Witch" of the team, in that he's almost scared of how powerful he could be. I'd like an episode where he, under stress, snaps and makes everyone on the team see a vision of their fears. But he would be over-determined in defeating his opponents--he is a god, after all--and would become the yin to Gorgon's yang. He would be the "little brother" of the Secret Warriors.
3). Ultron
Who is he? Well, everybody knows Ultron. Created by Tony Stark, killed by Vision, destroyed Sokovia, blah blah blah. While some have speculated that Vision might have actually absorbed Ultron into the Mind Stone, in the comics there are almost always multiples of Ultron running around everywhere. His future is uncertain, but he's bound to return at some point, once again hell-bent on annihilation.
How should he be used? When Ultron first attacked the Earth, he started at the top of the food chain with the Avengers, Earth's Mightiest Heroes. He knew that they were a formidable force and planned for every possible outcome, including his defeat, as we would see that he made copies of himself and saving them somewhere either off-internet, or deeply hidden within it. Someone--villain, hero, it matters not--discovers this file, and unleashes a new Ultron upon the world. This Ultron learns from the previous incarnation's mistakes, and decides to start this time at the bottom of the food chain with government agencies, eventually stumbling upon Phil Coulson's S.H.I.E.L.D. It's been explicitly said that S.H.I.E.L.D.'s mission is the protection of human life. Ultron's mission is the annihilation of human life. The story writes itself.
Budget is the main concern here, as recreating the expensive ten-foot tall CGI robot we saw in Avengers: Age of Ultron is downright impossible. They did make a Terminator TV show with robots, but I don't know how costly that would be on this show. I would keep that kind of stuff to a minimum and have something much more emotional be the focus: Ultron hacks into Deathlok's hardware and acts through him. This wouldn't be a repeat of season one, where Deathlok is forced to obey Hydra to save his son. I would compare this more to Hive's control over Grant Ward's body--how much of Mike Peterson is still in there? Can he ever return?
2). Eric O'Grady
Who is he? Eric O'Grady is the third Ant-Man, following in Hank Pym and Scott Lang's footsteps. More importantly to us, he's a former S.H.I.E.L.D. agent and perverted douchebag, who might have a heart of gold somewhere deep down. He also has connections to Damage Control who are getting their own TV series soon.
How should he be used? Just as Skye didn’t start out as Quake, Eric O'Grady doesn't have to start out as an Ant-Man, though we may or may not get there over time. It's his character, his personality that would make a good addition here. There's a lot of crap to explore with him.
For instance, though he's a smartass that seems to only care about himself, what would break him? What would change him? It would help the rest of the cast too. Let's take Daisy for example, who has had 3 love interests over the course of the show. What would it be like to see her be pulled to the wild side (think Daredevil with Elektra) by him? It's just a thought, and it's just scratching the surface, especially if he eventually did get the Ant-Man suit.
1). The Real Mandarin
Who is he? You know I had to go there, folks. There is only one true Mandarin, and he thinks all you bitches are irrelevant. To refresh your memory on his history, rewatch the All Hail the King one-shot. Basically, he's an ancient Middle Eastern warrior king that the Ten Rings worship. He has ten rings that each possess a different power. And I really want him in Agents of S.H.I.E.L.D.
How should he be used? A lot of fans want the Mandarin to be preserved for an Iron Man 4—if and when that happens. However, if they cast a big enough actor, the Mandarin could easily make the shift from show to movies should that happen years later (think Powers Boothes’ Gideon Malick).
I perceive the Mandarin as an ancient supervillain who craves worship and power from the whole world over. I would have his ten rings be extra-terrestrial in nature, having been based on the power of the Infinity Stones. Perhaps he even has to go on a journey to collect or recollect them all. Still he would crave more power. One reveal I would have is him finally acquiring the Jericho missile, and the Secret Warriors having to reassemble to keep it from launching/exploding. Having him as an opposite of Gorgon might be fun to explore as well.
Those are my top 5 picks, CBM--let me know who YOU would like to see in the comment section!