Fox has seen immense success with Deadpool and Logan, its R-rated superhero films. In addition to receiving rave critical reviews, the two have achieved great financial success, earning $783.1 million and $615.6 million respectively. Marvel Studios president Kevin Feige also praised those R-rated films for pushing creative boundaries and taking risks in an interview this past April. In that same interview, Feige said that there aren't any plans for an R-rated Marvel film -- but now, he seems far more open to the possibility, at least in a recent interview with AlloCiné.
Responding to a question about whether or not audiences would see bloody and violent heroes in the MCU, Feige said that although they are not currently planning an R-rated film, "I don't think it's out of the question. When I started at Marvel seventeen years ago, the Blade franchise was doing very well. A lot of people didn't even know that it was based on a Marvel character because at the time they sort of hid the fact it was Marvel. So, not out of the question, but not something we're working on right now."
In his statement, Feige makes reference to the Blade franchise. The series was one of earliest box-office successes for a Marvel character and followed the titular vampire who hunted other vampires and protected humans. The first film in the franchise, Blade, was released in 1998 and carried an R-rating. The movie's two sequels, Blade II and Blade: Trinity were also rated R.
(via MCU Exchange)