I just feel like Zoom comes down to four people, based on the content of the show. This is pulling from many aspects of the show all at once and my apologies ahead of time for any confusion.
1 Henry Allen- This could very well be a red herring. I think Henry Allen's disappearance, early in the season, may indicate Zoom's introduction. On the second Earth, he may have witnessed his wife's death. Henry remembers Barry's part in it as the Flash. An Earth-2 version of Henry Allen lived to destroy Barry Allen, once he had learned that he is the Flash. He decides in all life times for not stopping his wife's death that he must kill his son on all of these Earths. A prison incident may have been the death of Earth-1's Henry Allen and the introduction of Earth-2’s Henry Allen.
2 Joe West- An Earth-2 Joe West may have taken the mantle as an alternative Flash. Jay Garrick may not have saved Barry Allen, on Earth-2. Barry, caught stopping some crime may have been killed. He can blame Jay. Joe might not be close to Iris, is divorced, and never encounters his son, but connected to Barry. He and Jay Garrick may not be the best friends after Barry's death. Jay is also a less effective speedster, on Earth-2; hence, Harrison Wells experiments on other Meta people. Joe turns to Harrison Wells. Joe takes on this power. Power corrupts. He becomes Zoom in order to change history.
3 Barry Allen- This would be the most obvious choice. Jay Garrick and Barry Allen exist on Earth-2. The two Flashes stop crime and perfect the world. Barry connects with Harrison Wells and they relate. Jay Garrick and Harrison Wells have a terrible connection. Barry distances himself from Jay and only listens to Harrison Wells. Barry gets faster, more powerful, and understands the speed force. In the end, Harrison Wells experiments on Barry until he is as fast as he can make him. Barry still wants to save his mother, though he is at his peak speed. He still can only corrupt time. He wants to go beyond this. Harrison Wells proposes that Barry steals speed from others in different dimensions, or even from Jay Garrick. Speed corrupts and Barry becomes evil, and cruel. He jumps in time to take Earth 1's Barry Allen at his peak to save his mother but really to control the nature of all time.
4 Jay Garrick- It would only make sense. It just seems like something is off about Jay Garrick. He is sick. He lost his speed to Zoom. He is the absolute good of his character. This is the character of Jay Garrick. Though, what if Harrison Wells split Jay Garrick into two components? Jay Garrick does not like Harrison Wells for his experiments. Jay Garrick may have trusted Harrison Wells to help increase his speed against enemies. In the experiment, he creates a divide in Jay Garrick. All his anger, aggression, bad attributes, and negative components become a darker half of him. The Jay Garrick that the team knows may only be his good components, but is a weaker, slower, and more fearful side of the same character. This would explain why Jay Garrick knows this Zoom character so well.
This is all guessing. It could very well be a darker, more powerful version of reverse Flash, Wally West, Iris West, Barry’s mother, some other character, or even Eddie Thawne. Eddie Thawne did die and enter the portal. Eobard Thawne dies around the same. The two characters meet, in death, could make a pact. Eobard links Eddie to the speed force. In order for Eddie to do something important, Eobard may have convinced him to become this fearful character in order to escape death, change history, and make something of his life. This may have allowed him to become Zoom. The hope is the character has enough depth and gasp factor to entice fans of the Flash to fear Zoom and prepare for the future effectiveness of that character.