I fall into the crowd that sees Indiana Jones as pretty similar to James Bond, so I don't really have a problem with a new actor taking on the hat and whip. Sean Connery was the original James Bond, and probably the best James Bond, but the subsequent Bond actors still brought something to the character, and many of my favorite Bond movies do not star Connery.
At this point I would be more interested in seeing a new actor as Indiana Jones than a 70-something Harrison Ford. Indiana Jones isn't exactly high art. These are fun action-adventure movies, so I don't see recasting the lead character as that big a deal as long as the movie is still entertaining.
Now with that being said, who do you go with as the new Indiana Jones?
I think John Hamm would be a good choice. He's 42 right now, so a few years older than Ford was in The Last Crusade. I don't really see that as a big deal though since the guy's in decent shape and doesn't really look all that old. The important thing is that he possesses the natural charm and charisma that you would need to play Indiana Jones. He has a solid range as an actor, and he's also capable of projecting the intelligence and worldly wisdom that would be required. So we have a guy who can be charming, funny, serious, heroic, witty and intelligent, plus he kind of looks the part. All around a solid choice.
My wildcard choice would be Robert Downey Jr. Yes, he's older than Ford was (he's 48), but it's still a far cry from where Ford is now, plus he doesn't really come across as that old a guy. Once again, if things like charm, charisma, wit and intelligence are how you would describe Indiana Jones, RDJ undoubtedly has the ability to convey all of those in spades. He's been Tony Stark and Sherlock Holmes, and I really don't see Indiana Jones as that big a stretch. Indy has a knack for getting himself into one mess after another. There is a sort of "out of the frying pan and into the fire" approach to the story, and I think RDJ would excel in that those types of situations. You probably couldn't afford to hire him though.