I think they should go way back in Indy's past and get creative with the bad guy.
How about the heavy kid from The Last Crusade? The one that kept blowing the bugle all the time and brought the law to Indy's house. He could have grown up to be a collector of fine objects while having created many ties to underworld forces along the way.
He could be bent on revenge for all the abuse he got as a kid and stands in the way of Indy getting a certain artifact of great importance that in the hands of ignorance could kill many, etc.
How about the heavy kid from The Last Crusade? The one that kept blowing the bugle all the time and brought the law to Indy's house. He could have grown up to be a collector of fine objects while having created many ties to underworld forces along the way.
He could be bent on revenge for all the abuse he got as a kid and stands in the way of Indy getting a certain artifact of great importance that in the hands of ignorance could kill many, etc.