Deadpool is hard to do a sequel for, because what makes him great is how he plays off of other characters.
Unless Fox is willing to fold Deadpool into the greater X-Universe, the character's mileage is limited.
And this comes from a guy who loves Deadpool's supporting cast and villains. I'm thrilled we get Weasel and Blind Al(!), but unless they do T-Ray (Sure) or LL&L (no fucking way) in the next movie, they're in a shitty place.