Ember Moon

Ember Moon was actually in the running to win the Money in the Bank ladder match for the ladies, but a decision was reportedly made last minute to have Bayley win instead. While either choice was fine in my book, it still means that Moon has yet to win a title on the main roster.
Bayley just won the SmackDown Women's Title at MITB, but before that, the title had already changed hands three times since March. While a couple of those were due to the main event of WrestleMania, it still stands to reason that it is a title that could see as many as four more women win the title in 2019.
'The Hugger' will likely have a decent reign, but Flair is already looming with her nine title reigns. She already won the title twice this year, albeit for extremely short reigns. Bayley will likely have to fend off a line of competition including Flair, Moon, Mandy Rose and many others in order just to retain her title.
Because she'll likely have a big match at SummerSlam, she could feasibly lose it there or shortly thereafter. Few women outside of Flair (before this year) and Ronda Rousey have had lengthy reigns as champs, so Bayley will likely not finish the year holding the title.
Moon is certainly a possibility since she was already in the discussion to win the briefcase. The SmackDown ladies' division is one of the deepest divisions in WWE so it shouldn't be a shock if someone like Moon hoists a title in 2019.