For fun, I was thinking about bad guys from outside comics that would make fitting enemies for Diana.
Gozer the Gozarian from Ghostbusters
A (made up) Sumerian destroyer god/being that must be summoned via a ceremony involving possession and demon-dogs and can take any form it wants. You know, if you take away the comical aspects of the original Ghostbusters movie, the actual plot involving Gozer is a pretty straight supernatural horror story that I think would suit Wonder Woman very well as she contends with the Keymaster and Gatekeeper and the cult surrounding them before facing Gozer itself in whatever form it chooses.
Senator Armstrong from Metal Gear Solid
A corrupt US Senator trying to orchestrate political upheaval so he accumulate power and turn the world into his own libertarian fantasy land. There's potential political intrigue that would test Diana's diplomatic skills, and just listen to his big motivation speech and tell me this isn't a guy who would totally worship Ares. He also injected himself with nanomachines to give him superhuman strength and durability, and as he demonstrates in the game, he isn't afraid to throw down.
Pennywise from It
An ancient, shape-shifting Eldritch being that preys on children by exploiting their fears and eating them. Naturally, Diana would despise anything that hurts young kids, and if you've read the book, Pennywise is terrified of anyone who isn't afraid of it. So their battle would be less about a fist-fight and more about her trying to protect children from it.