The main villain of Captain America: Brave New World wasn’t really the Red Hulk, as promotional materials suggested. Although that is indeed who Cap has a big, final act fight with. We think it would have been silly to keep Red Hulk out of marketing. No, it was the long-forgotten MCU character Samuel Sterns, played by Tim Blake Nelson. His brain mutated thanks to gamma exposure, Sterns is now the classic Hulk villain the Leader. (Even if they never do use that name).

The actor returned to the role after almost two decades. And we think if Marvel Studios kept Stern’s return under wraps, a lot of Brave New World’s visceral impact might have remained. Instead, they spoiled a potentially huge twist years before the movie came out, we think to the film’s detriment.
Marvel Teased the Return of Tim Blake Nelson in The Incredible Hulk, Later Abandoning That Plot Thread

Marvel Studios introduced Samuel Sterns in The Incredible Hulk, a seemingly helpful scientist who reveals his own greedy agenda. The movie included a scene of Bruce Banner’s blood infecting his brain in the 2008 film, thus teasing his emergence as the Leader in a potential sequel. But when that follow-up film never happened, that storyline languished in the MCU vault of unfinished plots and post-credits stingers. After so many years, not even the biggest MCU fan expected to see the Samuel Sterns teases ever pay off. We think most viewers would have been genuinely shocked. Assuming they remembered the character at all, of course.
Marvel Studios Spoiled the Return of Tim Blake Nelson at D23 in 2022

Marvel Studios decided to publicly announce the return of Tim Blake Nelson with the full cast announcement for Captain America: Brave New World at the D23 Expo in 2022. So the reveal that his character Sterns would finally be “the Big Bad” was pretty much ruined in that moment. As a puppet master villain, there was no way that was not the direction he was taking in the MCU. Now if they had not structured the film to build up Sterns as a big reveal within the story itself? Then it wouldn’t have been a big issue. But the film has Sam investigating a mystery many fans already knew the answer to for much of its runtime. Yes, the ‘how” and “why” don’t get a reveal right away, but all of that is secondary to “who.” And the audience already knew “who” the Captain America villain was before Captain America did.
It’s Hard to Keep Marvel Casting Under Wraps, But Deadpool & Wolverine Proved That It Can Be Done

To be fair to Marvel Studios, it is extremely difficult to keep characters and cast a secret. We recognize that fact. Especially when making movies with huge fanbases like these. Movie sets have leaks, and scoopers online have made that spoiler culture pervasive. But Marvel Studios has also proven that one can keep casting announcements and characters a secret, as with Deadpool & Wolverine. That film had a metric ton of major cameos, most kept under wraps completely until the movie dropped. So it’s not impossible. It’s just very hard to do. Would the Sterns surprise suddenly make Brave New World a five-star movie? Well, no. But that big twist might have had fans genuinely shocked, and impressed them with how well Marvel played the long game. That itself would have been a positive. Sadly, it’s a creative decision they simply dropped the ball on.