I'll keep this short: I'm all in favor of George Lucas and Steven Spielberg continuing the Indiana Jones series, and I submit that Nathan Fillion should take over for Harrison Ford. I'll extol Fillion's virtues in a moment, but let me first briefly explain why I'm arguing that such a sacrosanct role should be recast in the first place.
According to most legendary accounts, Spielberg and Lucas hatched the plot for the Indiana Jones movies after one of them (Spielberg, I think) said that he'd love to direct a James Bond movie. Lucas had the idea for the Indiana Jones character, and they launched into Raiders of the Lost Ark from there.
Clearly, Harrison Ford has only so many years left where he could play Indy as a hero credibly (or at all). Lucas told Fox News that he has an idea for a fifth Indy movie that focuses on Shia LeBeouf's character. I haven't seen Kingdom of the Crystal Skull yet, so I can't speculate on whether LeBeouf (or his character) could take over the franchise, but let me say this: If LeBeouf were to assume Indy's mantle, I'd rather he simply be cast as the new Indiana Jones.
In fact, I seem to remember rumors that Paramount once upon a time wanted to turn the series over to River Phoenix, and may I say that such an idea breaks my heart. Phoenix made an excellent Indy, and I love the idea that each lead actor in the series would have appeared in three movies, and that in every third movie, the opening sequence would feature a young Indy story that would introduce a new actor to the role.
I'm sure I never actually heard that rumor, but what a nice thought. In any event, that brings us to my main point: That Nathan Fillion should be the new Indiana Jones. My argument has nothing to do with LeBeouf. I enjoyed his performances in Transformers and Holes, and I actually think he'd make a fine Indy. But Fillion has already established his whip-cracking credentials as Capt. Mal Reynolds on Firefly. Granted, Reynolds has a lot more in common with Han Solo than Indiana Jones, but Solo and Jones have always been blood-cousins, and not just because Harrison Ford turned them into icons. Indy is what Han Solo might have been if he had been given more of a chance growing up -- educated, brilliant -- but he's just as much of a scoundrel sometimes, except when the chips are down. Then he saves the day.
That said, I submit that Fillion is a much stronger actor than Harrison Ford (or LeBeouf), and that he would fit comfortably into the Indiana Jones role. He could honor Ford's legacy while making the role his own. I suspect that Fillion would bring even more humor to an already funny character.
To wit, I think that Indy's most "Mal Reynolds" moment comes at the beginning of Temple of Doom, when he gloats "Nice try, Lao Che!" only to find himself in a doomed prop-plane owned by the very same Chinese gangster. Indiana Jones and Mal Reynolds are both adventurous bumblers -- and I love 'em both.
So if you're listening out there, Paramount, George Lucas and Steven Spielberg -- think about moving forward with the Indiana Jones franchise, and consider one of the best actors working today as your new leading man: Nathan Fillion.