Johnson's men begin their campaign in New Zealand against Argentina on Saturday, September 10, before other group fixtures against Georgia, Romania and Scotland.
However, with just over three weeks to go until the kick-off, Johnson has revealed that has not identified his strongest starting 15.
"We're pretty much there with our 30 [man squad] for the World Cup, but in terms of the starting 15, there's still a little way to go," he told the RFU website.
"The two different sets of forwards have done pretty well."
Johnson plans to name his 30-man squad to travel to New Zealand on Monday, August 22.