I vote sometime between the 1950s and 1970s. While I definitely want more of Themyscria and the Greek mythology (possibly a trip to the underworld?), I feel it necessary to show Wonder Woman really confront institutional sexism and the patriarchy headfirst in a way she did not in the first movie, and I would rather do it in a striking setting.
While sexism obviously still exists (looks toward the White House), the context of her doing it during the real beginning of the modern women's liberation movement could be the most illuminating, narratively. Plus if she perhaps returned to Paradise Island after WWI ended (which would explain why she didn't do anything in regards to World War II), then we could get a movie of her really coming to see the role of women in society and how she can change that, which was her original mission statement upon creation. Add in some Greek mythology and maybe a bit of espionage spy movie flavor (it worked or X-Men: First Class) and you could have a really good movie.
We will get plenty of time of her in the present with Justice League and its sequels, and probably Wonder Woman 3. I hope that Patty takes her time building up to that though and first really explores some of the possibilities of setting the film somewhere in the mid-20th century. It would allow her to even say more about today in a way that could be less controversial and thus still very commercial while doing everything else fans want done.