Cory will be back but trevor is long gone, unfortunately. There were tensions between the actor who played him and basically everyone else in the cast and crew. He wrote a blog about it a long time ago and it sounded like him being a cry baby for the most part. Season 7 without the fuck twins was by far the worst season of the show...but there were a lot of reasons for that.
I hope the time they've had off (the boys) gave them some perspective because pretty much everyone agrees that 7 was the worst. I mean, it was still better than 95% of television...but it was too outlandish, too slick, it came off as more of a parody of TPB instead of an original chapter to fit in with all the others. The success of TPB in the future will depend on whether they have figured out a way to stay true to the heart of the show and still satisfy all the new viewers Netflix will bring. I'm a fairly pessimistic person though, and just going by season 7 and the films, as well as the swearnet content over the past year or so...I am not convinced. At all. I hope I'm wrong.