There's really no question that Deadpool is now the movie that's going to carry the Fox X-Men franchise forward into whatever it is the series has planned next. We don't even know what the main series even has planned next. Having said that, don't expect to see a lot of your favorite X-Men pop up in the sequel. Writer Rhett Reese says that while they probably could add just about any character they want in a Deadpool sequel, bringing in those top tier names would make a Deadpool movie feel a lot less like the sort of movie the first one was.
You kinda don't want a Ferrari in Deadpool unless it's for a real purpose in the sense that Deadpool is a gritty, grimy, smaller thing, with characters you aren't as familiar with. It's a little bit like lovable losers. It's not the A-list of superheroes, necessarily. ... The occasional cameo or appearance by an A-list type person could be really funny and/or dramatic. So we have an eye open for those things, but we don't want to just put people in it because we can now, if that makes sense.
When Deadpool was first being made nobody had any idea whether or not it would be successful. While there was clearly a vocal audience that wanted to see this movie, it was nearly impossible to know how that would translate at the box office. As it turns out, it translated quite nicely, making Deadpool one of the top films of 2016 and one of the highest grossing R-rated movies ever made. The sky is now the limit for Rhett Reese and Paul Wernick when it comes to the sequel, but the pair tells Collider that they don't need to fly quite that high.
A good deal of the charm of the original Deadpool was how differently the movie felt from the average superhero material on the screen. As Rhett Reese says, it was a grittier type of story. He admits that if they wanted to make Deadpool 2 a $200 million film, they almost certainly could, he doesn't anticipate Fox would say no, but that's not the movie thy want to make. They'd much rather continue Deadpool on the path that they put him on, which means pairing him with smaller heroes that won't be as familiar to people.
Having said all that, Rhett Reese doesn't close the door on cameos for major characters altogether, so it will be interesting if we hear of anything like that happening when Deadpool 2 actually gets underway. Overall, however, it would appear that the sequel will be more of what people loved about the first movie, and there's really nothing wrong with that. The biggest issue is that we still don't know when we'll be able to see it.