To me it won't seem like a "real" MM generation unless they have four members.
I would like to see them let A. D. Nagisa jump ship and join. She can be the member who will get down with her bad self no matter how silly she looks doing it. They already have a built in joke were they can be in the middle of a MC an a member will ask her to get her a bottle of water.
I would also like to see a jailbaitus extremus member join, someone the variety show hosts can (pretend to) perv over.
Of course they should get a yankii girl to fill the void of Reina leaving.
And finally, they should get another utahime because there's only so far that Riho and Sakura can take them by themselves.