I like the Cerberus theory, basically Blackbeard would've eaten a mythological Zoan, model Cerberus, which would allow him to have 3 fruits in total due to the "3 heads". I think it's cool because that would make Teach a Zoan/Paramecia/Logia hybrid, which is kinda cool, unseen and original for a final(?) OP boss.yup, thats why i believe that blackbeard has tow brothers inside him, i forgot the medical term for that, but it would explain everything, he uses the yomi yomi no mi to put the df powers he steals into his brothers, since they cant eat on their own.
his flag further evidences this, as it is extremely similar to the flag of the decalvan brothers, twin captains of a crew allied with whitebeard, but his flag has 3 skulls rather then 2, meaning that he and his brothers are triplets rather then twins, this also means that he is probably looking for a third DF, if he hasnt already found one.
That doesn't seem to be the case anymore though... I mean, whether BB has 2 or 3 brothers doesn't mean anything, we were already told several times that he's been hunting DF users in the New World... if he could only have 2 or 3 he certainly wouldn't be at it for so long...
Maybe it's just me, but even before the timeskip I got into my head that Blackbeard would steal DFs and give 2 of them to each of his crew members to make them super OP, and with everything we've been told post-timeskip I'm inclined to believe that that's what he's doing. I mean, all the SHs having to face a member of the BB pirates, each with 2 DFs seems like a pretty cool setup for the "final fights".
So if it's proven/shown that BB is actually giving these powers to his crew, then it probably has nothing to do with him having brothers or anything of the sort. Otherwise that would mean that BB will have many, many DFs for himself, and somehow I just don't imagine Oda going that far.
I mean, Teach is already making "history" by being the first person to have 2 DFs, why go further than that and make him have 10, 20, 50? I think it would be just too troublesome to keep track of, and eventually the Yami/Gura ones would end up losing their importance.