If I remember correctly,Sayu is older than Reina,right? So if it goes by age,Sayu is going to be the next leader.. But,I think the possibilities of Reina becoming the leader is higher than Sayu's,because Tsunku thinks Reina has more potential of an leader more than Sayu,right? But again,I'm not Tsunku so I don't know.. XD
I was hoping for Reina to be leader, too, if only to boost the image of Morning Musume as singers. But Tsunku appointed Sayu, so I guess he has his reasons. I just hope they don't involve Reina graduating in the near future since they didn't even make her subleader.