Sounds excellent; I'm second poster, to clarify things. Some more ideas, if you're still seeking some brainfood, could be the use of multiple personae in addition to 'effects' (a gun effect, a baton effect, crime scene tape, etc) all diving on his psyche. Perhaps he starts out seeming mostly sane and slowly descends into the heap of crazy we're familiar with at the end?
As for the cases themselves, that's more tricky. Solving them should be as much about thinking like Adachi as it is using normal solutions. Let's say that the intro is that Adachi's 'hub', which is the midnight channel reception area (or perhaps that little apartment? or even a very calm but oddly quiet and foggy Magatsu Inaba?).
Would the story be more linear than YN? As in, cases proceed semi-chronologically? Or, can you enter each fractured world at any time? I really like the idea of being able to tell your progress through the midnight channel; perhaps you're looking from the outside into the 'real' world however, and as you progress, Adachi's head get's less foggy and the midnight channel becomes more foggy, because that's how he sees things. I have loads more ideas, but no more rambling for now. If no one else, I'm looking forward to this, and'd be glad to throw out ideas and whatnot.