Most people are speculating that the next single from BTW to be released will be Heavy Metal Lover. Gaga has not officially claimed that it will be the next single. Fans simply jumped to the conclusion because Gaga sang a few lines on New Years Eve - that doesn't mean anything. She did the same with past songs that were never released as singles. Also, Amazon is simply releasing the song as a stand alone single, that to does not mean it will be an official single, the same was done with other singles, ie Dance in the Dark was released as a single in Australia and a few other countries but it was not made into an official single or video.
Gaga will presumably release Electric Chapel as her next single. While talking to Perez Hilton, she shared that she was going to name her next tour "The Born This Way Ball in the Electric Chapel" on Perez Hilton All Access. Also, it is important to pay close attention to her next few performances because she may incorporate imagery from the next video into her wardrobe and stage sets, like she did with You and I at the Sydney Town Hall last year, ie, she wore the Mermaid tail during the performance of the song.
People should not jump to conclusions about what the next single will be, as Gaga herself has said, things change day to day with her. One day she may want to do one thing, the next day, it will be something else.