1. Introduction of Death, Deadpool's other on-again-off-again girlfriend.
2. Introduction of the white and yellow text boxes and accompanying internal head voices.
3. Increasing of Deadpool's insanity to match the level he's at in the comics (could be the end result from the final fight in the first film when he got stabbed in the head. We saw that it invoked a hallucination. Who knows what else has come from that blade being in his head).
4. At least one scene showing Wade making a whole bunch of pancakes.
5. Showing a fight where we see how unpredictable Deadpool can be, even going as far as for him to pull out a weapon out of thin air ala Hammerspace (as he's been known to do such crazy things in the comics).
6. Hugh Jackman guest-starring as Wolverine, with a scene similar to this from the video game: