To put it simply, my theory is that during the Wano Arc, we will find out how Blackbeard is able to use two devil fruits. I have thought this since the reveal that Marco is a doctor.
Although this could always have been inferred, based on his devil fruit being a phoenix, which are mythically linked to healing properties. But before now, we had never seen him explicitly being referred to as a doctor, or administering medical care, or anything like that. It also wouldn’t be that farfetched for Whitebeard’s right-hand man to be a doctor either. After all, in the One Piece world, Trafalgar Law is a pirate captain, but also a doctor.
So if we make the logical guess that Marco did not simply become a doctor after Whitebeard’s death, but has long been a doctor, then it would be a strong connection with one of the other plotlines related to Marco: what he knows about Blackbeard’s body. According to Marco, there is something strange about Blackbeard’s body, that could explain why he can wield two devil fruits at the same time.
Of course his comment also implies that the whole crew is aware of the same info, so it does not necessarily mean Marco had to have been the one to find out, or the one to say it, but I am assuming that in any case, he understands it better than the rest of the Whitebeard crew). Being a doctor would provide insight about this. As a doctor, he could have examined Blackbeard while he was once part of Whitebeard crew, or perhaps treated him after taking injuries. With these opportunities and his medical knowledge, he may have been able to find out something about Blackbeard’s body that makes him unique.
*Theory by pierre_x10