I have this gut feeling that Death Parade will culminate with Decim’s judgment of Onna. In the OP there’s a shot of Decim holding Onna before the elevators. It seems as if he has to choose where to send her, but is hesitating.
We know that the Chavvot is connected to Onna and can infer that it may be a more tangible, compressed version of her memories. The image of the book on top of a stained glass memory panel in the OP definitely makes it plausible.
Onna’s memories factor into her judgment. In Death March, there’s a scene where Nona tells Decim that “After drawing out the darkness in [humans’] souls as much as possible, you pass judgment. In order to create the necessary extreme conditions, a person’s memories are absolutely imperative.”
Cue flashback to Onna and boom- ya boy Decim’s had an epiphany. Nona asks him what’s wrong, he says nothing, and she smiles and lets Decim go.
I think it’s at this point that Decim realizes what Nona is doing- setting things up so that Onna will slowly regain her memories. This creates those “extreme conditions” that will be necessary to judge Onna. During her time as an assistant, Onna’s actions and response to her memories will most likely serve as factors in her judgment.
Before letting go of Decim’s shirt, Nona smiles because Quin’s test has done what she wants it to- get Decim to realize how Onna’s judgment period will go. Initially, Decim didn’t understand what the Chavvot Roulette was, but now he understands its purpose.
I’m not sure if Death BIlliards counts as part of Death Parade’s canon, but if it does then we know it must come after episode 5 as they are using the new roulette board.
At the end of the short, Decim asks Onna if she’s learned anything, which could possibly mean he’s trying to gauge her reactions and thoughts so he can ultimately judge her.
However, will Decim ever pass final judgment on Onna? Does he have the right to do so? Who has the right to judge, and in what circumstances? These are just some of the questions Death Parade seems to be asking in regards to Decim and Onna’s relationship.