Harley Quinn wouldn't be the most awful option. I mean who the fuck is Doomsday? She's only a relatively new character, being made for the Batman animated series, which I never watched, so hadn't even heard of her till a couple of years ago. Surprisingly for a something made for the cartoon she's actually a really interesting well conceived character, with this tragic abusive twisted love story with the Joker, although to the casual observer Harley Quinn is 'Sexy Joker' which seems to be a popular enough concept that has assured she is well known. Obviously in the awful bloated mess of a movie that was Suicide Squad, the interesting chemistry and backstory was rushed and all over the place, but even then Harley Quinn was the one element a lot of people seemed to like.
There is a well known rumor that the ride was supposed to be themed to Suicide Squad at one point but they changed their minds because it was an unpalatable theme after last year. Creatively it would have been an awful idea anyway, why anyone thought that adding a ride themed to an outdated abomination of a film starring villains outside the brand spanking new Villains themed area would be a good move is, and always will be, a total mystery to me. It's a ride that should have always been themed to one of the big DC heros, with Flash, Wonder Woman or Jason Mona's Aquaman being very obvious and good choices. Now it's the wrong colour for them, (no one cares about a comic where Wonder Woman wore pink, if you make a Magenta Wonder Woman coaster it'll need a better explanation than that to keep the feminists happy) so what to call Australia's best coaster that happens to be magenta? Could do much worse than Harley Quinn, even if she is a villain in a hero area and is represented in Arkham Asylum.