I would like to see Sallah once again. He was great in Raiders and Crusade. From what I know, John Rhys-Davies would be happy to return, although I read somewhere, that he rejected cameo in Skull.
Interaction between Willie and Marion would be great to see especially as Indy is married to the latter.
What about Short Round? As an adult he could be the similar adventurer as Indy and they could go for an adventure together.
Marcus Brody won't be back, but Charles Stanforth could return...
I definitely don't want the deaths of Marion and Mutt. If they won't have anything to do in the script - just send them to other side of the country.
I know it won't happen, but it would be great, if they would bring back Henry Jones Sr. They killed him off in Skull, but let's just say, that he was missing and presumed dead or something like this.
On a side note - imagine the following scene:
Action takes place in the 60s and Indy sees the poster for one of the James Bond movies. He stops for a while, scratches his head and mumbles to himself "This guy looks just like my father..."