I remember hearing in an interview that Ryan Reynolds wouldn't be involved in the film
unless the movie was 100% faithful to source material. I think Reynolds realises what the fans
want, being a fan of deadpool himself. Although that's not entirely up to him what happens with the story, i think Reynolds is too important to risk him leaving the movie so i think the spin off should be faithful.
Seeming that he's already involved in the film and it is in draft writing stages, i think the script being produced at the moment would be a close origin story of deadpool in the comics but also adapting to how deadpool was portrayed in x-men origins wolverine.