The Flash doesn’t pick each season up right where the last one left off, so whatever left turns happen in the Season 4 finale will likely inspire another big change of pace for Season 5, and that could mean anything from different timelines — some people are dead certain that Mystery Girl is Dawn Allen, possibly from 1,000 years in the future — to dimensional shifts to a newly vengeful Speed Force.
Considering how few breadcrumbs we’ve gotten about her, it’s still really hard to say where things are going.
Because if Mystery Girl serves as a narrative impetus for a future, as opposed to just a side-character curiosity, then her true identity could end up proving true one of the more seemingly outlandish theories kicking around.