Now that Wilson is Captain America, ...CA 4 can do the Serpent Society, Madbomb, Madam Hydra and Red Skull in a hottub, turning Presidents into lizards, etc.
I like to add some 'Falcon' themes to CA4 (If Marvel Studios make that movie);
NIGHTSHADE. She first showed up turning Falcon into a Werewolf, and brainwashed a bunch of SHIELD agents. And she did all that in a bikini, thigh-high boots and twin Afros (Then she wore a lab suit and turned Steve Rogers into CapWolf, not fun).
Stoneface. A forgotten New York gangsta', probably because he's been ex-patrioted and sent to Africa where he kidnapped Leah and tried to throw Falcon and the Black Panther off a cliff.
Leah, Ralf, and people living in Harlem. Well, the people living in Harlem rallied together to fight the Commie-smashing Captain America to rescue Falcon. Ralf hates Sam, Sam likes Leah, Leah likes Ralf, Leah hates Captain America but likes Falcon. Lots of tensions.
Hey, I wanted to mention Nightshade. I thought there was a lot of story and fun that can come from this character (I liked that line she said, "Oh Pooh!")
What about Machinesmith? He's a traditional Cap villain, and his control over machinery would make him a nice foil for FalCap, who relies heavily on his mechanical wings and mechanical (in MCU) Redwing.
Throw Zemo in to keep the waters good and muddy and you've got yourself a movie!