I find it interesting that most people suggest Haruna because of her leadership qualities and most people suggest Mizuki because of seniority.
Personally the fact that they were appointed as co-sub-leaders has always been a rather out of the ordinary move to begin with. Haruna went over the rest of the 9kis to get that spot. Management and publicity seems to give both of them equal turns in some ways to be leader-like, though Haruna makes more of it than Mizuki currently... because of that I'm still of the mindset that a Co-Leadership is going to be the most likely. But they also might have done it so that they could easily give Haruna sub-leadership without the 9kis being upset later.
Haruna's age may also be a factor in that management thinks she'll be the next to graduate so she'll be appointed leader next and then Mizuki and Ikuta will get leader/sub
Personally I want to see Haruna being a leader, she just has that leadership quality to her
![XD XD](./images/smilies/XD.gif)