Aside from just looking a bit older, are there any other differences you'd like to see in Indy's look?
I've seen several people say that he should have a beard, and I agree that it could be a good look for him. Should he have side-burns? Or maybe a mustache (rather than a beard) would be appropriate for the time frame?
What about his wardrobe? Should the 70s time period (assuming that's when the movie takes place) have any impact on how he dresses?
Personally, I think they should keep the look pretty classic. Give him a beard, and give him some pants that fit a little better than those baggy ones he wore in KOTCS.