I just thought of something. What if Luffy did gave Zeff his leg instead agreeing to work as a chore boy at the Baratie? Would that mean his leg would be still be rubber when attached to Zeff or just turn to normal. If it's still rubber, that would open the possibility for a person to have more than 1 devil fruit ability. Maybe if the devil fruit user's body doesn't reject the transplanted limb due to blood or devil fruit powers.
For example, what if a person like Buggy cut someone's arm off who has a devil fruit ability that allows him to create glue. And then attach it to his body with surgery then only can create glue with that arm but his Bara Bara no Mi won't allow him to separate that arm since it's affected by another Devil Fruit ablility.
So what do you guys think of this bizarre theory?