Most characters are born Ds. However there are significant hints that D status can be adopted.
Ds are supposed to be the enemies of the CDs. However Lily was one of the 20 Kings, yet she was a D. Therefore Lily's D status must have been adopted. Furthermore you have panels like this, where Ace seemingly gives Sabo the D.
While Sabo never officially adopted the D, this at least hints that D status can be adopted. And I think fundamentally this makes sense given the nature of the will of D. Will of D is an inherited will. This is something that isn't tied to bloodline. Roger had the will of D, Luffy inherited Roger's will. However Luffy isn't tied by blood to Roger. This was something mentioned in MF itself. Even if you kill Roger's bloodline, his Will will live on.
I think that once Luffy goes to Laugh Tale and uncovers the meaning of D, there will be a lot of characters who will adopt the D. After all, Ds are the enemies of the CDs and a lot of characters hate the CDs.
Specifically, I think there are a few characters that the D would fit really well with. Those being:
Koby is along similar lines. He is Garp's protege, so I feel like Koby will get the D from him. I think that he will become KoDy.
Doffy might seem like an unusual choice, but I think it makes a lot of sense. Amongst the 20 families, there are only 2 that seemed to go against Imu. The first is obviously the Nefertari family, that adopted the D and decided against becoming CDs. However the DonQuixote family also has a history of going against the WG. First we had Homing leaving Mariejois, Rocinante dying to save Law(against Sengoku's wishes), Doffy becoming a pirate, and Mysogard attacking Charloss to protect Shirahoshi. For Doffy particularly, he seemed to have a grudge against the CDs, as they cast him out and were basically responsible for his horrible childhood. So I feel like it would make a lot of sense if, in the end, Doffy adopted the D and decided to go against the Celestial Dragons in the final war.
I think Doffy would become Donquixote D. Oflamingo.
Lastly, I'd like to nominate Kid. I think this one is probably the weakest, but I think it still has some merit to it. Kid is associated with Law and Luffy, both of whom are Ds. He was there when Luffy punched Charloss and was a major player in Wano. Hence I feel like there is a big chance he will be a major player in the final war, with his role likely being to 2v1 Imu with Law before Luffy faces him. Imu has immortality, Law can probably take away immortality. law has never done anything in a 1v1, so Kid will probably be his sidekick and help Law take away Imu's immortality. If Kid becomes a D, I think rather than it coming from Luffy or Law, it will come from his childhood friend Victoria, who will turn out to be a secret D. The real life Kid's name was William Kidd. However One Piece Kid only has one D in his name. When he adopts the D, Kid will become Eustass Kid D. or Kid D. Eustass.
Anyways that's just a few, but I think there could be others. Ultimately, there is no reason for the will of D to be as exclusive as it is, once the meaning of D is revealed. At that point, anyone who resonates with the D or who has ties to D members will adopt the D. This is the reason why the WG erased the meaning of D, they didn't want people to identify with it.
Ds are supposed to be the enemies of the CDs. However Lily was one of the 20 Kings, yet she was a D. Therefore Lily's D status must have been adopted. Furthermore you have panels like this, where Ace seemingly gives Sabo the D.

While Sabo never officially adopted the D, this at least hints that D status can be adopted. And I think fundamentally this makes sense given the nature of the will of D. Will of D is an inherited will. This is something that isn't tied to bloodline. Roger had the will of D, Luffy inherited Roger's will. However Luffy isn't tied by blood to Roger. This was something mentioned in MF itself. Even if you kill Roger's bloodline, his Will will live on.
I think that once Luffy goes to Laugh Tale and uncovers the meaning of D, there will be a lot of characters who will adopt the D. After all, Ds are the enemies of the CDs and a lot of characters hate the CDs.
Specifically, I think there are a few characters that the D would fit really well with. Those being:
- Sabo
- Koby
- Doflamingo
- Kid
Koby is along similar lines. He is Garp's protege, so I feel like Koby will get the D from him. I think that he will become KoDy.
Doffy might seem like an unusual choice, but I think it makes a lot of sense. Amongst the 20 families, there are only 2 that seemed to go against Imu. The first is obviously the Nefertari family, that adopted the D and decided against becoming CDs. However the DonQuixote family also has a history of going against the WG. First we had Homing leaving Mariejois, Rocinante dying to save Law(against Sengoku's wishes), Doffy becoming a pirate, and Mysogard attacking Charloss to protect Shirahoshi. For Doffy particularly, he seemed to have a grudge against the CDs, as they cast him out and were basically responsible for his horrible childhood. So I feel like it would make a lot of sense if, in the end, Doffy adopted the D and decided to go against the Celestial Dragons in the final war.
I think Doffy would become Donquixote D. Oflamingo.
Lastly, I'd like to nominate Kid. I think this one is probably the weakest, but I think it still has some merit to it. Kid is associated with Law and Luffy, both of whom are Ds. He was there when Luffy punched Charloss and was a major player in Wano. Hence I feel like there is a big chance he will be a major player in the final war, with his role likely being to 2v1 Imu with Law before Luffy faces him. Imu has immortality, Law can probably take away immortality. law has never done anything in a 1v1, so Kid will probably be his sidekick and help Law take away Imu's immortality. If Kid becomes a D, I think rather than it coming from Luffy or Law, it will come from his childhood friend Victoria, who will turn out to be a secret D. The real life Kid's name was William Kidd. However One Piece Kid only has one D in his name. When he adopts the D, Kid will become Eustass Kid D. or Kid D. Eustass.
Anyways that's just a few, but I think there could be others. Ultimately, there is no reason for the will of D to be as exclusive as it is, once the meaning of D is revealed. At that point, anyone who resonates with the D or who has ties to D members will adopt the D. This is the reason why the WG erased the meaning of D, they didn't want people to identify with it.