Not to be blatently obvious, but we have no proof Goodwin is the fifth Signer. Yes, he has the arm. But it's entirely possible something random happened and he got the arm.
On top of that, have we ever seen the guy with a deck at all? No proof he plays, certainly none that he has the dragon.
We don't know that the arm is Goodwin's at all, even though he does have his mechanical one and everything. Need I also point out, on the arm the birthmark was on the left arm, as opposed to the other Signers, whose birthmarks are on their right arms.
Just pointing out my arguments against Goodwin. It's not that I don't like the guy, he just seems a little too villainous to be a Signer (then again, Krieger apparently joined the Dark Signers and he didn't seem like the kind of person to do hat, so yeah.)