Because I'm lazy I'll just take what I wrote over at jpm, modify it slightly, and then repaste it here.
I don't think most of you understand the implications of the name "Diva". While most people tend to associate being a diva with having some measure of vocal ability, the true indication that one is a diva is the artist's stage presence and ability to control the audience. The word Diva is latin for goddess and so the name refers to an artist that recieves a god-like reverence. This is why Rika is in Def. Diva and not another h!p soloist like Kei, Kaori, Nakazawa, or any of the other myriad solo girls lacking work.
As for why Rika and not Miki or Takahashi or someone else in Morning Musume? Well, it's quite obvious that this group is intended for an appearance at kouhaku. If you don't know what kouhaku is, the I suggest you click this
link. Since Diva appears to be intended to be for kouhaku, then Morning Musume members can't be in Diva. Not because it would be unfair, but because each artist can only participate in one group. Thus, because Morning Musume will almost assuredly be on kouhaku this year, any members of Morning Musume are automatically out of consideration.
Also, I don't think it is prudent to suggest that Diva will be NUN + Rika. The name alone should tell you that the theme of Diva will differ significantly from NUN. While NUN was very up-beat and techno-rock inspired, Diva will almost certainly be far more epic in it's theme. Diva should showcase these girls in a very different manner from NUN and because of that, I think this group has the potential to make a much bigger impact than Gomattou or NUN and will probably serve as an excellent promotion for these girl's new, individual works that are still on the horizon.
In other words, I'm excited about the prospects of Def. Diva.
Ok, so maybe I didn't edit it, sue me