I know I'm not the first guy to theorize the identity of Zoom and I won't be the last, but hopefully I can attempt to be the most detailed. Everything in this article is pure speculation and I have no inside information. Now let's begin!
1.) The Storyline of The Season
In the first episode,"The Man Who Saved Central City" we see Barry grief stricken and questioning his hero status; feeling that Eddie and Ronnie were the true heroes and they deserved the praise. In episode six, "Enter Zoom" Harrison "Harry" Wells calls Jay Garrick a coward and proclaims that Barry is a true hero. If it wasn't clear, my point is that the storyline of the season is what it means to be a true hero and Barry accepting himself as that. Barry versus Eddie would allow Barry to resole any lingering guilt and affirm himself as the true hero of Central City. Bringing this season's storyline to a satisfying close!
2.) How and Why it Works
Before I see the comments saying "Rick Cosnett is a series regular on another show!" Let me tell you something...It doesn't matter! Unlike a lot of viewers, I wasn't expecting a Zoom reveal. Because I know Zoom is Eddie and I know he's a series regular on another show. So they decided to do the opposite of last year and reveal the villain's motive first. Then in the second to last episode we'll get the villain reveal. Exterminating the problem of Rick Cosnett being on another show! The "how" Eddie comes back is a little left field, but bare with me. When Eddie shoots himself Eobard disappears before Eddie dies(correct me if I'm wrong) meaning that Eddie only had to be "basically dead" to eliminate Eobard from the timeline. Also, where did his body end up? The breaches all lead to Earth 2! The color of the breaches are blue, Zoom's lightning/speed force is blue! In episode six "Enter Zoom" Harrison Wells says that Zoom used to be human. So my theory is that Eddie became a speedster that exists outside the flow of time, like a mix of Black Racer/Professor Zoom!
3.) The Motive
In the midseason finale Zoom tells Harisson that he simply wants Barry's speed. On first thought that seems like the exact thing Eobard Thawne wanted, because it is. I think the reason they want the speed is very different though, If Eddie is Zoom I think he wants the speed to rid the world of speedsters. The worst thing that ever happened to Eddie is meeting speedsters. The only people he's ever shown hurting is people that are in some way related to the creation of speedsters or is a speedster. He didn't hurt Linda, in fact he dropped her and let The Flash catch him. The fear the other villains have for zoom seems to only come from what he's done to Jay Garrrick! I know a lot of fans think it's Henry Allen, my question to those people is why would he hurt Barry? Why would he become Zoom?