Blood:Here are a few things that could definitely add to the investigation. Silhouettes from blood splatters can reveal the killer's general body shape, it wouldn't have to be super detailed, just a few squares of blood on the wall/floor that our detective can piece together. Blood residue on weapons and clothes would make cameras more useful, and it would help identify the murder weapon, so you don't have to guess which of the 8 knives did the killer use. If you have the weapon used to kill the victim, it shouldn't be too hard to tell that it's the right one. Blood should also make people who have interacted with it leave distinct fingerprints and footprints. This would give us a good idea of the route the killer took, to help find fingerprints, or a likely method of entrance .
Timeline: This is super important for murder investigations. We should be able to ask people where they were at the time of the murder. Sometimes they should lie, either because they were doing something shady or because they murdered someone. This will help us figure out if someone is suspicious, because we can catch them lying.
Witnesses: Recently I had a case where a woman was murdered with a knife in front of her partner. When asked if the partner saw anything strange they responded with "I saw someone with glasses, they looked kinda angry, at 9 AM". They should at least be able to describe them better, with their outfit, height, skin color, hair color.
Psychology: killers should have their own motives. Sometimes revenge, sometimes money, sometimes they are just crazy. Currently, every killer acts basically the same, with a bit of flavor text thrown in. The stalker doesn't fawn over their victim. The corporate hitman doesn't tie you up in red tape. The psycho doesn't have a cruncher filled with pages of schizo text.
Currently, I feel like we reach dead ends way too often and end up just having to brute force check every person in the building for fingerprints and stuff like that.