In my own headcanon, Blackbeard's background and plot would be related to the D. family, their relationship with devil fruits and the connection between these and souls and, therefore, will (or haki). He wouldn't be capable of developing haki, and therefore would go for three devil fruits that could grant him abilities mimicking it: the Yami Yami, for armament; the Gura Gura, for conqueror's; and the mythical model of the Hito Hito, for observation (because of enlightenment as related to "seeing and feeling beyond yourself"). These three would also represent the strongest fruits in their categories.
Of course, this was debunked the moment Teach was confirmed as an armament and observation user.
Of course, this was debunked the moment Teach was confirmed as an armament and observation user.
