The following contains spoilers for Venom.
For a time many thought Venom would end up being an R-rated release, but instead, it ended up with the PG-13 that is standard for nearly all comic book movies. With a sequel teased in the closing credits of the first movie, and the introduction of an even more violent character than Venom, one might wonder if a potential sequel could end up being R-rated. We can stop wondering, it's probably not going to happen. According to Producer Avi Arad...
The post-credits sequence of Venom introduces the audience to Cletus Kasady, a character from the Spider-Man comics who becomes the villain known as Carnage. While the comics' Eddie Brock isn't really the nicest guy, Cletus Kasady is an actual psychopath and when he comes in contact with the alien symbiote, some pretty vile and vicious things happen. However, that doesn't mean the sequel, if it happens, needs to be R-rated. Ultimately, the feeling is that showing the violence simply isn't necessary to the story of Carnage, because what makes the character interesting is his motivation, not his violent tendencies.
It's certainly true that Carnage isn't an inherently R-rated character, the Spider-Man comics that he was introduced in would have to be considered no worse than PG-13 themselves generally and they've been able to depict the character just fine more through the suggestion of his violence than actually showing it. Though comics have the benefit of not showing violent action. In a similar vein, movies can get by with the suggestion of quite a lot of violence in a PG-13 rating.
None of this is to say that we won't see R-rated movies within Sony's planned comic book cinematic universe. Collider asked if any of the various spinoffs could see an R-rating and while it seems clear nobody is going to go looking for an R-rating, if the feeling is the story requires it, then it appears Sony is willing to go that direction.
The word prior to Venom's release was that part of the reason that the film went with the PG-13 rating was to make it easier to set up a future confrontation with Spider-Man, a battle a very large audience would want to see. While it remains to be seen if anything like that will ever happen, it seems the stage will be set for it if it ever does, as Venom will be staying comfortably in the teen-friendly part of the movie theater.