It's gotta be Kakashi. We've already seen first hand how the fuedula lords of the Land of Fire deliberate when they want to nominate a leader, and age, experience and who-you-know comes into big play. For the present time both Sasuke & Naruto are eliminated due to their age. They almost said 'no' to Kakashi due to his age
So Kakashi's certainly is going to be next in line to get it
After that, some 10 or 14 years down the road it'll come down to either Sasuke or Naruto (most likely Naruto)
Although after all of this they really should do away with the whole ninja system, which should include kages; otherwise they've learned nothing about the root causes of war, pain and conflict