if u think about it all the hokages are linked in some way . the first hokage was the grandfather of tsunade, the fifth hokage. also there has been rumors that the first and second hokage were brothers or cousins. the third hokage was a student of the second hokage. the fifth hokage was a student of the third hokage. the fourth hokage was a student of jiraiya who in turn was also a student of the third hokage like tsunade. this leads me to believe they were selected based on their links or ties to the previous hokages. this makes the rumors of the first and second hokage being either brothers or cousins believable. based on this we can guess that the sixth hokage will be linked to at least one of the hokages in some way. so i would have to say kakashi as the sixth hokage because he is the sensei of naruto who is under the theachings of jirsiya who is linked to the third and fourth hokage. this might also explain why jiraiya was considered for the fifth hokage besides the sannin explanation. if in fact the sixth hokage did become kakashi,because he is the only jounin who is linked to any hokage,naruto would seem a likely choice for the 7th hokage.because he would be linked to the 6th hokage kakashi and the third hokage because of jiraiya and even possibly the fourth hokage if he in fact was narutos dad.
so u see that is my explanation but that is just my explanantion feel free to criticize