Could they have kept Margot Robbie as Harley?
like how Judi Dench was kept as M for Casino Royale (even though it made no sense whatsoever)
Could've just passed it off as multiverse stuff i.e. a Margot Robbie looking HQ existed in the 80s as well as present day alt universe DCEU
Gaga is quite big now in movies but post Barbie Margot would've been HUGE