I honestly cannot imagine a different kind of ending. Happy endings are reserved for the harem and purity routes, and the dark route should be what is considered an evil/bad ending. So the taste of the true ending should be bittersweet.
There has already been a canon ending, which is "Times New Roman". In it, Akira's alive, Ami got sacrificed to end the resets and time goes on. Apparently things are just fine for Yasu and, to some extent, Sana. But that's it, it's certainly not a "happy" ending for Akira, but well, he's there.
Just from the previews, I wonder if Ami looking shocked and Niki breaking down have to do with it, and why. Niki breaking down is natural for a couple of things that could happen with Noriko, from her getting laid, to finding out about other girls, to just a flashback. But what gives with Ami?
I think one thing that would shock Ami is Akira
not having sex with Noriko yet again. I think that, although the obvious punch would be punching Niki from having sex with Noriko - maybe the punching is towards Noriko again?