According to Frank Marshall, Indiana Jones and the Scriptwriting Brigade may be in full swing soon. He told Empire Magazine that waiting another twenty years to do another Indian Jones film was out of the question.

So what will be the premise? Will it be nasty Nazis? No one knows.
What is known is Shia Labeouf is a talker. He's the one who let the plan be known. Though it's not much of a plan, but fans may turn up the heat to get the film made before Harrison Ford is unable to swing from high places.
Want another Indie Jones movie? We do. Let's just forgo the psychic, alien sideshow. We want to get down and dirty.
Source: Empire Magazine