The writer/producer said that he is still considering the plot device for the sequel.
Lucas told Vulture that partner Steven Spielberg's claim that the concept was settled was not quite accurate.
"Actually, I told him I didn't have it yet," Lucas said. "I told him about the story, but I really haven't found the MacGuffin yet.
"I mean, I know what it's about, but I just have to find a MacGuffin that fits into the arena we're working in."
A term popularised by Alfred Hitchcock, a 'MacGuffin' is a device that drives the plot and draws the viewers' attention but has no true significance beyond this purpose.
Spielberg had previously told Entertainment Weekly: "We have already agreed on the genre of the fifth movie, we already have a concept in mind."
Harrison Ford has said that he would be "delighted" to return to the Indiana Jones franchise for a fifth film, but is not sure if the sequel will go ahead.