i don't think Haruna would be made a leader before Fukuchan. they will either give it to Mizuki, or to both girls. there's no precedent for co-leadership, but there was also no precedent for co sub-leadership, so i think it's possible. but most likely it will be Mizuki leader and Haruna sub-leader.
i actually find it really interesting that Haruna was given sub-leadership at all. i wonder what the reasoning was? she showed that much promise? the non-Mizuki members were too childish to be up for the sub-leadership job? they wanted a representative from both 9ki and 10ki? they'd just been such sticklers for seniority until then, basing off of major debut + age (which is why i didn't find Yuka/Tomoko or Mikitty odd) that i didn't see it coming. not counting times where the most senior member decided not to be leader, or they were graduating soon so weren't really in a good position for leader.
i'm for Mizuki leadership. they need both of their skill sets so i'm good with co-leadership too, but if i had to choose one. tbh though, no combination of members is going to be able to fill the giant leadership shoes Sayumi will be leaving behind.
it might be several more years before any of these (or future) girls mature enough to be able to really make the most of their leadership tenure, like even a Gaki level.