At the Cannes film festival this weekend, when asked about the possibility of doing a 5th installment of the Indiana Jones franchise, George Lucas replied that he'd first have to come up with "an object for Indiana to seek."
That he made no mention at all to the rapid deterioration of Harrison Ford's face suggests to me that Lucas is already planning on replacing Ford entirely with digital technology, if not some elaborate scheme involving a ruse and a carbonite chamber.
Anyways, seeing as Lucas is probably a fan of this blog (hey, if it's good enough for you...), I've decided to use my advanced knowledge of mythology and Wikipedia to compile a list of likely MacGuffin's for Indy to chase after the fifth time around:
The Pendulous Swingers of Kranak
The Sumerians worshiped Kranak, who was the God of all things that dangle and sway.