As I recall the plans are something like this:
Moa/Yui: well for now they have Babymetal so they are set for a while
Hana : She want's to continue singing/dancing, I could see her continuing with the wrestling theme probably collaborating with the wrestling association.
Yunano: She want's to model/act
EDIT: Actually there was an interview recently that have now been translated where they talk about it - >
http://babymetalfanclub.com/direct-interview-with-the-march-graduating-sakura-gakuin-members/As for new students, I very curious on how many members they will add, they never added four new members before and I'm not sure that it would work.
On the other hand they kinda need to do something, they can't continue with having more members graduate than they add.
I have a wishlist of new members, one is Hana's little sister
http://artist.amuse.co.jp/artist/taguchi_otoha/ another is Rico Sakurai from Si-Stella I think she's extremely good for her age but she's not with Amuse so it's extremely unlikely.
Rico Sakurai is the one being Su here