In their extended on-set conversation with Collider, writers Rhett Reese and Paul Wernick talked about how their script evolved in the six years from their first "final draft" to the actual script that was shot. While it's "70%" the same, that last 30% did see significant changes, from supporting characters to rating.
But one thing that will be brand new is the vaunted Post-Credits scene, a staple of superhero movies since Marvel Studios started adding them on with Iron Man. When the interview was conducted on set a few months ago, they weren't positive that it would get done, but teased something they thought would be a blast.
"There is a post credit thing that if hopefully comes together, It’s gonna be phenomenal," Wernick teased. Reese said it all depended on the behind-the-scenes work. "We’re planning it. It’s one of those ones that’s gonna require some doing, so it may not come together, and if so, I don’t know, we’ll either do a plan be or none at all. I think we should have one, I mean, we really feel like we should have one."
The first time Ryan Reynolds played Deadpool in X-Men: Origins - Wolverine things... didn't go well. But there was a fun extra bit in the post-credits where his severed head opened its eyes and the mouth was no longer inexplicably sewn shut. Maybe a scene that will directly make fun of that, if they haven't already earlier in the film?