I believe that Eustass may be a D. Just a theory, but i have a decent amount of tentative evidence to back it up.
1. "D's are connected by their fates". Tralfagar D. Water Law, Monkey D. Luffy, and Kidd all happened to end up being together in the auction house when luffy attacked Saint Charloss. (And on Sabody Archipilago for that matter) Also worth noting that rociante stated that D's are the natural enemies of "God", most likely referring to the Celestial Dragons, and both Law and Kid smiled when one was attacked. Since they are all rivals, they are fated to battle in the new world before the end of the series."Let's meet again in the new world" -Kid
2. A strange occurance i picked up on in the anime. Each time this has happened, something pertaining to the will of D has later come up relating to the particular instance. It was only noticeable in the anime, but it would be difficult to portray it in the manga. The first instance i noticed was in Mocktown when luffy unknowingly encountered the blackbeard pirates by chance. The comedic scene where luffy and teach showed opposite tastes in food could have been foirshadowing their future battles to come. After Luffy and Zoro were thrown out of the bar by Bellamy's crew, Blackbeard made his speech. "The age where pirates dream will never en d. Then he raised up his glass. As soon as he did this, the screen went black and white as if it were a manga drawing and depicted luffy and teach as such. At the end of this arc, the man's name was revealed to be Marshall D. Teach.Later in the anime, i noticed it again. When Luffy punched the world noble. Same thing with the screen. Later on, many arcs later, Rociante revealed the will of D to be the natural enemy of "God." (He also spoke about when he lived in Marijois, suggesting that "God" refers to the world nobles. Finally, Eustass Kid proposed to Law and Luffy his desire to meet again in the New world. Once again, the screen becomes like and manga srawing. In the dressrosa arc, Law was revealed to be a D. Does this mean Kid will become a D.?
It could be something entirely different, and it's just a speculation, but 2.5 times out of 3, it matches up. Not much of Kid's back story has been revealed, so it's a possibility. What do you guys think?