Both of Marion's first scenes in Raiders and KOTCS involve Indy and Marion bickering about the past and Marion's life being threatened by a villain.
Indy translating and working out Ox's letter is a lot like Indy working from his Dad's diary to find the second marker and the Holy Grail.
Both the Crystal Skull and the Sankara Stones have already been technically found by someone who isn't Indy. Indy for the most part, spends his time trying to keep both items away from the baddies or stealing it back from them when they have it.
Both Spalko and Mola Ram used psychic/supernatural things on Indy.
The Crystal Skull's full power really only works when joined with the rest of the Crystal Skulls and skeletons, much like the Sankara Stones being the most powerful when all the stones are together.
The powers of the Crystal Skull and the Sankara stones are not clearly defined, and can do more than one power all at the same time!
In both the Raiders and KOTCS ending, Marion and Indy walk away arm-in-arm to points unknown (alright, perhaps not completely unknown... a bar in Raiders and perhaps to a honeymoon in KOTCS?)