James Gunn’s rebooted DC Universe has moved at a breakneck pace since ‘Superman’ hit theaters last year, and the speculation machine around its follow-up has been running just as fast. Every casting update, set photo and stray comment from the cast gets picked apart by fans hunting for confirmation of who’s really playing who.

That speculation has been especially intense around Adria Arjona, whose role in ‘Man of Tomorrow’ has remained a tightly guarded secret since she joined the project. While trade reports initially pointed toward her playing Maxima, an alien queen from Superman’s comic book mythology, persistent rumors have suggested she could actually be stepping into...