Simon Williams, AKA Wonder Man. He's extremely powerful, a constant Avengers member, and is an important character in several big events in the comics-and yet, he likely will not be seen on screen. Most non-comic fans likely have never heard of him, so why is he important, and why should he make it to the movies?
1. His brain pattern was used to create the Vision
This part of the story was obviously skipped already, with the Vision having been created in Age of Ultron with no Ionic Avenger in sight. In the comics, however, Wonder Man and the Vision share a close relationship and refer to each other as brothers. In addition, Martha Williams, Simon's mother, even refers to the Vision as her son! As a result of the brain patterns, the Vision and Wonder Man are very alike in their thinking (usually), and even their taste in women...
2. Wonder Man is a close friend and lover of the Scarlet Witch
Same taste in women, indeed. Simon has dated only a few women over the years, but the love of his life is, and always has been, Wanda Maximoff. More than anyone else, he trusts and defends her no matter the circumstances. In fact, he only joined the Avengers Unity Squad featured in the Uncanny Avengers series to support her. He's clearly close to the team members, but what has he done that merits a spot in a movie?
3. Simon sacrificed himself to defeat Korvac
If you know anything about Korvac, you know he pretty well wiped the floor with the Avengers AND the Guardians-at the same time. After the initial beat down, Wonder Man stood up and was quickly joined by Cap in a courageous assault. Both he and Cap were killed by Korvac, but the Avengers and Guardians rallied to continue the fight. Though Korvac overpowered them, he realized the evil he was doing, and returned the heroes to life while willing himself out of existence.
4. Wonder Man was key to Baron Zemo's plot to destroy The Avengers - and that's when he became a hero
When Simon Williams was put on trial for embezzlement (which he was doing in a desperate attempt to save his failing company through off the books means), a mysterious lady showed up to pay his bail-for a price. Amora the Enchantress brought Simon to Zemo, who transformed him in to the super powered Wonder Man. Zemo revealed to Simon that he would die unless he went along with Zemo's plans to infiltrate and destroy the Avengers. When it came down to it, however, Wonder Man turned on Zemo and the Masters of Evil, and sacrificed his life (temporarily) to save the Earth's Mightiest Heroes.
So where could the Ionic Avenger enter?
Right now, it seems unlikely that he'll appear at all. However, if Phase 4 has a little "Initiative" era theme, I wouldn't be completely surprised to see him alongside Carol Danvers on a Mighty Avengers-style team. It's a long shot, but Wonder Man could bring both a unique look and interesting character traits to the MCU, especially if he goes pacifist as he did in the comics.