I think it's been pretty well established that Hollywood likes a proven story. Especially when they're going to put a lot of money into a project. Is that a completely failsafe sort of formula? Not necessarily. Look at the run of Biblical epics. Lots of those films have done very well. But then there was Ridley Scott's recent effort, Exodus - Gods and Kings. Solid director. Solid star. And a story that's a known quantity. But it really didn't perform that well.
As for rebooting Indiana Jones? I think I would prefer to just leave the franchise alone. Kingdom of the Crystal Skull was not a good way to end things. If it had ended with The Last Crusade? THAT would have been an exercise in going out on a high note. It was a solid Indy movie. But these film franchises are big money, and Disney wants to get their money's worth. So? It's reboot time.
I saw something about the Chris Pratt rumor a few weeks back. And I had seen something about Bradley Cooper being considered, too. I think they could both fill the role reasonably well. And I think from a story standpoint it would be interesting to take the timeline a little bit further back than Raiders and Temple of Doom. How about starting off with Indy in the 1920's? How about some of that backstory of him and Abner Ravenwood?
The key though? A good script. Time and time again we've seen plenty of movies that had good casts and great filmmakers. But give them a crap story, like Indiana Jones and the Kingdom of the Crystal Skull? Guess what? You're going to end up with a dogshit movie. Sometimes you wind up with a writer/director/producer who has so much clout that nobody will speak truth to power... like George Lucas. And maybe they've gotten lazy or unfocused and their script needs some re-writing or punching up.
And that phenomenon isn't exclusive to Indy 4, but to lots of movies in this CGI era of movie making. Go in with a ****** story and 9 times out of 10 you're going to have a ****** movie. Part of the problem is screenwriters and directors who write around things to specifically shoehorn in special effects. Everyone seems to want the same level of attention for their effects that something like the bullet dodge in The Matrix got. Get over it. Write a solid screenplay and let the special effects serve that story, not the other way around.