Good points here - there's one disguise that Indy wears with utter competence, because it's not really a disguise at all. It's when he goes into "professor mode", a la the banquet scene in Temple of Doom. I could imagine a scene where he goes into that persona to distract some high society folk so the sidekick can sneak past or steal something. Obviously you don't want to take that sort of thing too far, lest the sidekick be doing all the proactive work, but the idea of Indy going into Clark Kent mode in front of certain people could be fun in small doses and would be a sensible, non-annoying way of utilizing the age of the character.
Indiana Jones had a reputation that preceded in him the previous movies. Even Chattar Lal recognized the name. He's probably even more globally recognized at the age he'd be in Indy 5. Koepp could exploit that in a number of ways, and it's the one useful way I could see the events of Indy 4 playing into Indy 5. It would be nice if the last film had been less ambiguous about how Indy got to be reinstated at the end - did he become famous for discovering Akator, or did that never become public (the city was washed away after all) and it's just that the Red Scare blew over? The difference between those two scenarios is not trivial in terms of where Indy's reputation stands.
