We can say, the real mcguffin is the Kingdom and the Crystal Skull is an artifact that leads to that mcguffin, in this case a real place, this is a very typical structure for an adventure movie.
1) Hero learns about something and goes after it - the mcguffin.
2) The audience learns MORE about the mcguffin than the hero watching by scenes on the opposing forces, hence increasing the suspense.
3) The hero starts to know the real value of the mcguffin.
4) The hero gets the mcguffin.
In this case:
1) Marion goes to Indy with a strange Cristal Skull, brings Mutt along, Indy doesn't get to see the son very much (akin to Last Crusade).
2) We're introduced to some scenes in the soviet republics where they are making some weird experiments, and learn the mcguffin is dangerous.
3) The hero learns the real meaning of the Skull, as a key/map/portal to a lost kingdom.
3) Indy gets there, uses the mcguffin to open kingdom.
Thats what i think will happen in general.