It doesn't sound like a plot-less story... I think. I'm trying to wrap my head around this. You want to write a mystery story, but instead of developing characters, you're developing the cases that surrounds the characters.
Um... I think you'll need the characters to move the cases along, honestly. They're murder-mysteries, obviously you're going to need to leave some clues behind that will lead to the one character that did it. Like every good detective story, you'll have to introduce the characters along with the case, and make sure you introduce the character who did it without saying they did it until the very end. Of course, since I never wrote murder-mysteries, I could be dumb in this department and that it's possible to write murder-mysteries without focusing on characters.
You can do what you want about characters. If you're going to use characters you made-up, you're going to have to have some character development, even when you're using other fictional characters. Because some people won't know who the heck the characters are otherwise I don't know who Detective Conan is, and unless you make the cases so intriguing that they're more potent than the characters, you're going to confuse readers. So... it's not a plot-less story. You can have it to where these collections of short stories actually all come together at the end, like they were the result of some crime lord or something. You know?
That's what I got from this thread, so... yeah.