I mean, im not saying it HAS to be Luffy, but it being Teach makes little sense to me because:
1. We've known for years Shanks and Teach hate each other
2. Why would Oda hid the name of the pirate if Teach is the obvious choice
It just makes little sense as an author to lay out the obvious and then NOT specify the obvious if it IS in fact the obvious lol.
Edit: This is, in fact, why Red Herrings exist....