I have spent the past week studying murder mysteries and how they're constructed.
One thing about me as a person is that I hate compromising an idea due to personal limitations. For this reason, I always try to research what I'm creating thoroughly rather than steering away from genres I don't know about. I love documentaries (but I'm a lot less keen on docudramas. Either make it a period drama or a documentary! Surely the facts in themselves are fascinating enough to not need the justification of some bad acting!?) and tend to devour information.
Murder mysteries are something I admire but have never really considered making myself. They are actually quite challenging to make well. You need to leave clues so the conclusion doesn't come out of nowhere, but without making the solution too obvious. I love laying out red herrings and coming up with motivation for characters, it's something I'm used to doing in comics, so a mystery is just that turned up a notch. You have to make it seem like everybody could feasibly be to blame at first. The hard part is not getting contrived or descending into farce.
The fun thing about making this first person and having you visit suspects personally, is knowing that you could be sat in the room with a killer. I think I'd want minimal music and sparse environs for this, like 'Portal', which would make sense on a space ship. I want the player to feel like they're alone with nobody to trust. Even though they have friends on the phone, those friends are thousands of miles away.
The Gap Year girl, Miranda, I would deliberately write as sympathetic and friendly. People latch onto a friendly face in a desolate environment, so it just makes it worse when she's the accomplice of the murderer and not trustworthy. The characters you have to work to gain the trust of make better friends. I guess the moral is that sometimes people who are a little shy of making friends at first make really trustworthy, loyal friendships.
One idea I like would be to have a little minigame area, like when you visit Dan, you can play retro arcade games with him, and gradually he relaxes and speaks to you while you're playing the game. I think it'd be good to be able to play the game a few times seeing different outcomes and conversations. There's no way you could see everything in one playthrough, but you could have a playthrough where rather than focusing on solving the case, you just hung out with and learned all about a particular character.