Blackbeard Will Get The One Piece
I have suspected this for a while.
The most upsetting thing that could happen later is that Luffy seemingly defeats everyone he needs to and then has a clear path to the One Piece only to find out that he was cleverly tricked by Blackbeard.
I imagine it will go something like..
"Yay!! Luffy won! Now we can save the world!"
Then Luffy looks around and realizes something isn't right. He then yells at his crew saying "He's already there!!"
and they ask who Luffy is talking about, and he responds "Blackbeard."
This sets the stage for a final showdown between him and Blackbeard, and Blackbeard will say some shit like...
"Zehahahaha! Luffy!! You're too late. I've already begun- yadda yadda."
This sort of thing would be in true Blackbeard-fashion.