How about Nov 2019? Or June, August or Sept 2019. Movies can do very well in August (Suicide Squad), Sept (It) or Nov (Frozen).
Geoff Johns and Patty Jenkins have been working on the script for a while. June 2020 is too far away. Strike while the iron is hot, right? Well, maybe early 2020.
Hindsight is 20/20. What if The Mummy, Cars 3, Transformers: The Last Knight, Despicable Me 3, or War for the Planet of the Apes were big hits?
As it happened, the biggest problem for Wonder Woman this summer wasn't the "competition" - The Mummy or Cars 3 or Spider-man: Homecoming.
It was Dunkirk, from the same studio. Losing a ton of theaters and 3D screens thanks to the Chris Nolan movie. Wonder Woman was still plugging along even after the release of digital until it lost half its theaters no thanks to Stephen King's It, from Warner Bros./New line.
So... if the sequel is anything like the original - great reviews, word of mouth, amazing LEGS - it'll be a big hit REGARDLESS of its release date or competition.
It may or may not be #1, but so what? Losing to Star Wars is no shame.