My ideal ending would be...
nobu x hachiko
shin x reira
nana x ren
If Hachiko had broken up properly with Takumi, there wouldn't be the awkwardness of being around the Black Stones. She'd still be close to Nana O. She wouldn't have to compete with another woman (Reira) over Takumi after marriage. (He even cheated on Hachiko.) Takumi has "grounded" Hachiko, but I don't think their unhappy marriage is worth it when he doesn't put Hachiko top-priority. I don't think Takumi will ever love Reira in that way, and Reira should be able to forget him with Shin.
Now that Ren died, (not to sound cruel, but) I now also support...
yasu x nana
I think that Yasu would be the next-best-pairing with Nana O.
So guys: List your ideal pairings, and tell me why!