9. Colonel Steve Trevor
Man of Steel featured a big government presence, and with Suicide Squad on the way (which will likely mean that A.R.G.U.S. have a big role in the DC Cinematic Universe), using Wonder Woman to expand on this would be a smart idea.
Military man Steve Trevor is the most sensible choice to help make this a reality and the fact that he has also been a love interest for the character over the years adds another interest dynamic. It would also be good to have a prominent “human” character in the movie as he could serve as an entry point for viewers to demonstrate how a God-like being such as Wonder Woman is seen through the eyes of a normal man who understandably falls in love with her.
Trevor is a great character, and as he later serves as a government liaison to the Justice League, that could help tie this movie into a cohesive shared universe.