With Guardians of the Galaxy vol. 3 off the table for now and its release date gone to another big tentpole already, it seems that Marvel Studios may have decided on a slate for 2020 -- and it is arguably their biggest gamble in a while.
Following Avengers 4 in 2019 and whatever reshuffling of the Marvel Cinematic Universe it brings, That Hashtag Show claims Marvel will fill its two remaining 2020 dates with Black Widow and The Eternals.
The timing makes sense; both movies have reportedly been in development for a while now, and Black Widow already has key components of its cast and crew in place. The Eternals is a property that may have ties directly to Avengers 4, since Thanos's in the comics is an Eternal himself.
A supposedly-leaked synopsis from Production Weekly explained Black Widow, which will apparently be in theaters on May 1, 2020, this way:
At birth the Black Widow (aka Natasha Romanova) is given to the KGB, which grooms her to become its ultimate operative. When the U.S.S.R. breaks up, the government tries to kill her as the action movies to present-day New York, where she is a freelance operative. The standalone film will find Romanoff living in the United States 15 years after the fall of the Soviet Union.
In spite of taking place several years before Iron Man 2 and the character's first onscreen appearance, the movie will still star Scarlett Johansson. Like Captain Marvel it will take place in the past, which means it will theoretically not speak directly to the events of Avengers 4. This also makes it a logical first movie to announce, as any returning characters can be cast and spotted on set without anyone claiming that it "spoils" any elements of the upcoming super-sequel.
Very little is known about The Eternals, which is supposed to be coming to theaters on November 6, 2020. The film will be directed by Chloe Zhao, and a synopsis that has been floating around the internet (seen below) suggests it will take place millennia before anything else in the Marvel Universe.
The story of ‘THE ETERNALS’ is set millions of years ago when the cosmic beings known as the Celestials genetically experimented on humans, creating the super-powered individuals as well as more villainous off-shoots known as Deviants. The two groups went on to battle each other throughout history to see which would eventually become the ultimate race. The story involves the love story between Ikaris, a man fueled by cosmic energy, and Sersi, who relishes moving amongst humans.
This might end up telling the story of the construction of the Infinity Stones, which would give it ties to the MCU, but in all likelihood those ties would not become evident until after Avengers 4 was already out anyway.