Here are a few theories on who I think "Zoom" really is. I also think that regardless of who it really is under the mask Zoom's real name should be Hunter Zolomon just like the comics version, but Hunter Zolomon will be the name of a doppelganger of an Earth-1 character.
Joe West (Earth-2)
Detective West has been Barry's surrogate father since Nora's murder and loves him like the son he never had, until learning about Wally. Now imagine him running around killing people left and right without regard, stalking people to inspire terror into them and torturing whoever he wants for his own amusement. It would be very strange to see Joe as a complete psychotic murderer. Also I have looked at scenes between Barry and Joe and then Barry and Zoom, and Joe and Zoom kinda look the same size.
Oliver Queen (Earth-2)
They have confirmed that Oliver on Earth-2 is dead, and Robert Queen is Green Arrow. FLASHPOINT!. Anyway Oliver is said to be dead but again looking at scenes between Oliver and Barry, then Barry and Zoom and Oliver again kind of looks like he could be Zoom's size. This would also be an interesting turn of events because Oliver is one of Barry's best friends and the thought of seeing Oliver trying to kill Barry is twisted.
Henry Allen (Earth-2)
This would be a good twist. Barry's biological father sent to prison by Eobard Thawne for murdering Nora, Henry spent 15 years in prison before being declared innocent. In the comics Henry is killed in a prison riot, so glad that didn't happen in the show, but wouldn't it be sad if something happened to Henry maybe even Zoom kills him and then Zoom unmasks himself as Earth-2 Henry. This would present a challenge to Barry, especially if this is the case and Henry does die, because Barry would essentially have to fight his father. It would also be awesome because John Wesley Shipp did play Barry Allen in the 90s series and now he'd be playing an evil Flash.
Eddie Thawne (Earth-2)
Barry and Eddie weren't completely friendly to begin with because of their feelings for Iris, Eddie even admitting he felt threatened by Barry at first, but by the end of the first season they grew into close acquaintances and genuine friends. Then Eddie kills himself to erase Eobard from existence which. along with Ronnie's death, impacted Barry deeply as well as Iris who clearly does still miss him. It would be interesting for both Barry and Iris for Zoom to be Earth-2 Eddie as I think Barry would find it difficult to fight him knowing it is Eddie mainly for Iris' sake.