I don't understand the difficulty with the MacGuffin. Isn't the whole idea of the MacGuffin that it just drives the plot without being anything particular? There are plenty of historic "treasures" that have intrinsic value without having a religious or supernatural connection.
To me a good Indy movie needs lots of good set-piece stunt, a scenery chewing villain, good comedic moments, exotic locales, and a few brawls. A world shattering MacGuffin isn't needed. A huge diamond, buried treasure, giant jade statues, personal writings of an important historic figure, all would work.
For example, Indy could find a relic in South America that refers to something buried in one of the pyramids. He goes there and finds that Napolean took it when he visited the pyramids. So he tracks around France to find it. But it was taken by a Brit after Waterloo and stashed in Wales somewhere. Who cares what it is exactly (evidence of transatlantic trade earlier than we thought, Ra's staff of power, recipe to really good beer), the point is that Indy has decided he wants it, some else wants it as well, and they travel to lots of cool places fighting to get it. Not terribly "epic", but it could make for a good flick if the writing and direction was sharp. I'd rather have frequent lesser adventures than one epic in scope journey every decade.
I also agree that Speilberg needs to hand over the direction to a younger, hungrier director.