It's still up in the air... whether or not Dennis (played by Glen Howerton) will return for the next season.
Howerton's character departed last season to be a father to his bastard child.
According to IndieWire-
The gang from “It’s Always Sunny in Philadelphia” took part in a panel discussion at the Vulture Festival in Los Angeles Sunday afternoon, but they’re keeping a tight lid on what’s coming for Season 13.
Though early in the discussion, co-creator Rob McElhenney said they were preparing to start up the writers’ room again, but would not say whether Glenn Howerton will be returning for the 2018 season.
“We’re getting back to the writers’ room in February ,” McElhenney said. Later in the discussion, he said production is slated to start in April and the release date would be in the fall of 2018.
They were more tight-lipped when pressed about Howerton’s return.
“Eh, damn it,” Howerton said when first asked about coming back. Despite the crowd’s cheers for him to return, he did not commit either way. “I will say this: All joking aside, I love these guys and we have an absolute blast working together. We’ll see,” he said.