Indiana Jones and the Kingdom of the Crystal Skull was undeniably terrible. And honestly if they're smart they won't do another. However this is how I think a proper sequel would go.
1) Be relative to Harrison Ford's age. Treat him like Sean Connery in the Last Crusade. With Mutt (so to speak) doing the heavy lifting. Which brings me to my next point:
2) Recast and rename Mutt. Have him demand to be called Henry or something, and Indy insists on calling him junior. See below for the recast.
3) It should be 25% of Harrison/Mutt. More of a narrative for a flashback to during the war on one of Indy's adventures.
4) It should be 75% the adventure in the war. This is Indy in his freaking prime. Fighting Nazis and stealing their s**t. This is what everyone loves and wants to see. This can't be Harrison though he's much too old. See for below for the recast.
Marian-Mila Kunis (during wartime)
Let's face it, she has the feel that the original Marian did.
Marian-Original (during Harrison's time).
Older Indy - Harrison Ford
Mutt/Henry - Joseph Gordan Levitt
I think he could take the character in a much different direction than Shia, which is a good thing.
and for the big recast:
Young Indy - Michael Fassbender
He could nail this role. (X-Men First Class bar fight, anyone?)