There's so much fascinating stuff in Oppenheimer's life beyond just the Manhatten Project which, you know, is cinematic enough as is. Just tinkering around with some of the other potential main characters:
Jessie Buckley as Kitty Oppenheimer - Robert's wife, a *former* communist struggling with addiction
Bobby Cannavale as Edward Teller - A Hungarian immigrant (one of the so-called "Martians"), the "father of the hydrogen bomb," would later testify against his former boss before Congress and advocate for expanding the nuclear arsenal
Jack Black as Leo Szilard - Another "Martian," Together with Enrico Fermi invented the nuclear reactor, fought against the use of nuclear weapons during the war and spent much of his later life advocating for the destruction of the US nuclear arsenal
Rhys Ifans as Enrico Fermi - An Italian immigrant with a Jewish wife who fled fascist persecution, together with Szilard invented the nuclear reactor. Came to work for Robert at Los Alamos. Opposed the use of nuclear weapons and would later testify before Congress on Robert's behalf.
Elizabeth Debicki as Jean Tatlock - A communist, Robert's lover before and after the war. Institutionalized for chronic depression, commits suicide during the war.