Reynolds, in all his interviews, is pretty adamant about the fact that he's only on-board for playing Deadpool for as long as they keep the movie faithful to Deadpool's origins - cancer, fourth-wall breaking, and the whole nine yards.
The movie!Pool (which I'm sort of growing fond of now oh God) could actually be pretty easily transformed into our favourite merc with a mouth - there were several hints dropped at the end of the movie to that effect - and considering this is Marvel, I doubt they'd have a problem handwaving some of the stuff away. ;)