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Your theory is very interesting.

1) You are correct on how Teach has become an antagonist, however a better one is Sakazuki, for he killed a character and made the protagonist doubt his powers. As for having two devil fruits I believe the Yami Yami no Mi has more secrets than Teach let on for Jimbe said he was searching for other powerful devil fruit users meaning Teach can have more than the two.

2) It's a move he dedicated to his older brother, perhaps the spirit of his brother helps, or the determination of Luffy protecting everybody ignites it, sure the anime showed his brother when doing it but I really don't think the fruit wore off onto Luffy.

How do you know Teach never ate Whitebeards fruit, the amount of time for when a fruit gains the devil fruit power is a matter of seconds thanks to Slimey in Punk Hazard, besides they covered up Teach and Whitebeard so you couldn't see what he was doing, they had to hurry didn't they. As for your transfer part, you maybe half right there kid. But we can only wait and see what Oda has planned.

Interesting, but I think Blackbeard attaining 2 devil fruit powers was just because of the ability of his fruit so I don't think Luffy or anyone in his crew will eat it. I think Luffy will be like Shanks where Shanks was carrying around the Gomi Gomi no Mi fruit before Luffy ate it so my theory (and I don't expect this to come true) is that Luffy will also keep it around like treasure.

To be honest I am not completely sure about Blackbeard but his powers are not already normal for a logia user, since, usually logia users can turn into the element they obtained but Blackbeard has not been shown to have the ability to turn into darkness; their is properly more to this power than meets the eye. But I think this way since it has been said in the manga that he has spent a long time looking for this specific fruit and there is properly a special reason for it.

Also I don't think it is possible for someone to have the ability to eat 2 devil fruits but if their was another way to obtain the devil fruit powers then yeah, I can see Luffy with 2 devil fruits but otherwise I don't think Luffy will have 2 devil fruits. I think Blackbeard is suppose to seem like the final boss for not just Luffy but for the entire One Piece world.