The X-Men are kind of insane to the point where half of the insanity of the series can be explained by saying "because X-Men." Deadpool is in the X-Men world, but has thus more or less escaped how insane the universe actually is. The sequel isn't going to be able to avoid that, as they are bringing in Cable, who is probably one of the most insane comic book characters ever. The short version is that he's the son of Cyclops and a clone of Jean Grey from a potential future timeline that he was transported to as an infant, and that's leaving a lot out. One of the things that made Deadpool so successful is that it managed to appeal to a rather wide audience despite being an obscure character that only comic fans had heard of.
In an interview with Collider, writers Paul Wernick and Rhett Reese are already thinking about how to approach this character while walking the line of being faithful to it, but not weighing down their fun action comedy with exposition.
What's it been like writing Cable?
PAUL WERNICK: It's intimidating because we have to find his voice, and we don't have an actor yet so we don't know who we're writing for. Most screenwriting happens without cast in place, but sequels it's a little bit different and it's likely that part will get cast, and we'll get to continue writing it and honing it for that actor, which I think is a good thing. There's been so many diverse people that have been brought up as possibilities for Cable, and who have not been brought up but we're considering. And that, much like with Ryan [Reynolds], that decision will greatly impact his cadences, his rhythms. And we'll certainly make him like he is in the comics, and we'll certainly be faithful, but I do think that voice will inform us quite a bit.
RHETT REESE: I think authenticity is so important. I think part of what made Deadpool such a success, I think we're going to continue that authenticity.
WERNICK: So with Cable, he's got such a convoluted past, and such a convoluted origin story that I think we're going to try to leave that, not mysterious, but there are a lot of twists and turns, cloning and all this stuff where you go, "Oh my God. How do we get that across in a two hour movie?" I think we're going to distill him down to his essence. It will be authentic and faithful, but it's not going to include the 18,000 details if you were going to read a Wikipedia page about Cable you'd roll your eyes.
This is about as 'safe' of an answer as they could have given. Deadpool was comic accurate but it was also a movie that felt like everyone involved had a lot of love for the character and the world. Despite the fact that the sequel has lost director Tim Miller three fourth of the people involved with the original are still there and still love these insane characters.
We can hope that Reese and Wernick don't have to wait too long for the role to be cast; that man is going to have to put muscles on top of muscles somehow and get a lot of pouches to get properly into character.