With Dr. Vegapunk in the latest chapter mentioning that Devil Fruits might be born from people’s dreams, a lot of theories have started arising, and I too have one. Mine is a bit shorter than most others I’ve seen here, but should have some major implications.
Blackbeard is a character with bunch of mystery surrounding him, but one thing known about him, is that he supposedly doesn’t sleep. This would in turn mean that he doesn’t dream.
It might be that Blackbeard can use multiple Devil Fruits because they aren’t his own dreams, he’s literally stealing the dreams of others.
Going a bit further with this, Devil Fruit Awakenings might be when the users’ dreams align with the ones that creates the devil fruit itself, which would mean that Blackbeard would be unable to awaken any devil fruit.
*Theory by DeadPieGamer