As with many others, Riho was the one who I thought that perhaps could be a new ace, if only Tsunku would actually grab her up.
THIS! TRUTH!
Honestly, MM can't rely on Aichan to carry the group forever during concerts/live performances. As much as I/many of us dislike the thought of it, we all know that someday Aichan
will also graduate from the group. If/When that happens, we all know that the group would be MAJORLY screwed if there wasn't anyone who had the ability to be her successor (not necessarily in terms of leadership, mind you, but in terms of performing on stage). Now I hate to say it, especially because it's been hard for me to find something to make me NOT like any of the current members, but really, none of them would have been able to take that center spot (in terms of performing/on-stage presence) that Aichan has owned for the last few years, at least not fully and not on their own. MM has been in major need of a new "ace" type member, someone who can "take the ball and run with it" so to speak, if for no other reason than to keep the group from becoming "Takahashi Ai et al".
Personally I see potential in at least 3 out of the 4 new members to become future "front girls". However out of them, IMO Riho's going to be the one who one day will be "given the ball."