Now people are excited and equally as curious about what this new story arc will bring, but I just want to give my 2 cents on who I think Goku Black is.
Now Goku Black isn't a clone or some sort of copy, he's not even Goku being under mind control. Actually I think Goku Black actually is Goku. Now let me explain what that means. The timeline that Future Trunks is from technically would be considered the "original" timeline since him time traveling actually changed the way things were suppose to be. Now in Trunk's time it's implied that Goku actually uses instant transmission and kills Frieza and King Cold than soon after before the Androids come dies of the heart virus. Now because Goku is the one to kill Frieza and King Cold and because he dies from the heart virus thus eliminating all the experiences the Goku we all know and love had that would have a significant effect on his growth and development as a person. Now after he dies since Goku is still considered a hero he of course keeps his body and spends the rest of his time for the next several decades training and getting stronger and stronger. Eventually since no one in other world challenges Goku he becomes jaded and he desperate seeks of someone or something to push him. Goku eventually encounters some kind of Evil Demonic God(like Demigra for example since both him and Black Goku have the green potara and they make it a point to really emphasize the earrings in the promotional poster). After this God defeats Goku, Goku being the person he is immediately intrigued and desperately wants to train with him to eventually attempt to surprise him so he agrees allow if Goku agrees to sell his soul or do something for him for him. Goku of course questions this but eventually Goku not having a reason to protect anyone now that he's dead and and being doomed to not being challenged as a fighter is easily persuaded to agree; or Goku fights him, loses but the Evil God sees Goku's incredible potential and that leads to him than manipulating Goku to be his underling, so anyway, they train and eventually Goku learns God Ki and perhaps further abilities since he's trained with him consistently for years by this point. Eventually, the Evil God and Goku after Goku has reached a certain level of power break out the other world reek havok everywhere including Earth which leads to him almost killing Future Trunks, which prompts Trunks to go back in time again where our story arc begins.