A Detective Investigation mod? You would join The Imperial Guild Of Investigation, quests would be somthing like, a Body was found in the Elven Gardens District. You have to find the subtle clues to put the murderer away,But it wont be easy you will ahve to use your mind and the killer isn't just going to let you find them on purpose.I am thinking There would be a Chapter of the guild in each city but the HQ would be in the Imperial City. I want it to be an extensive mod, not just a one quest deal.

There is already a mod like that called http://www.tesnexus.com/downloads/file.php?id=13134, but sadly it's bit unfinished.

Oh, well I will make mine better, and finish it.

also his/hers is making a new investigation firm, mine will be an already existing one.

Yeah, I would definitely like to see something like this.

The only problem I see is that you cannot have a scenario where you'd have to find the killer among the usual citizens. We already know who belongs to the vanilla game, so as soon as we'd see a new NPC walking around, we'd probably know who is the murderer. Your criminal would have to be hidden in a dungeon or something like that.

If it was called something along the lines of "Veridius Investigations", and not "Imperial Guild of Investigation", I'd be very interested in it.
Private detectives are much more interesting than official police departments. And anyway, the guards of the various towns act as crime investigators already in many vanilla quests; it would be weird to have a new Imperial office that does the same thing for non-vanilla quests, if you know what I mean.

Oh, and: If you go for a private investigator who hires the player as his employee, you need a group of street urchins like the "Baker Street Irregulars".

segvirion: That won't be a problem. Usually, you'll have many suspects anyway, so that there isn't *one* new NPC that has to be the criminal. And in a modded game, there are often loads of new NPCs as well.

Unless I add Many none quest related NPCs to the world, I wasn't really planning on having the killer out in the open.

YES!

That is so funny, I have A Study in Scarlet sitting on the table in front of me right now.

sounds alright...I hate it when quests don't give you enough information to find where to go though!
I know some people have some good deduction skills and are able to figure such things out, but I lack that skill a little, and so I need either lots of hints, or detailed hints. (I can be fine without a questmarker in 95% of the scenarios)

However, as mentioned segvarion, some NPCs would stand out...though...even though I have put (in all gameplay across both platforms) atleast 500+ hours in playing around in Oblivion, I STILL do not know every existing NPC.
A good way to get around it is add new NPCs all across the board, so that way, even those who have photographic memory or have devoted ungodly amounts of gameplay time to this game to remember all the NPCs the new ones - whether part of the murder case or not - will delude them into easily deducing who might the perpetrator be.

and, even still on that topic, I doubt every killer would think the same: "Act natural, stay in town, no suspicion"...I mean, some might get scared as it was their first kill and run to a nearby cave, or perhaps, to help create their alibi, they don't live in town at all but operate in small forts/caves/villas outside of the city.
So, in that case, the player could not even use deduction skills by seeing who in town it may be, because the killer will not even be in town. Sure, there is a chance the player may come across a non-vanilla place...but honestly, once again, who has played this game enough to have every location memorized? I haven't, and combined with all my files from xbox - pc have not discovered every vanilla cave/fort ruin/camp/aylied ruin/villages/priorys and so forth!
And if you do not want the player to even beable to find the answer simply by finding a non-vanilla location, than use a vanilla cave/fort ruin/ect! there are plenty to go around and be shared by other mods, so it wouldn't hurt to use one for a small boss fight or whatever you plan on doing with the murders.

anyways...just some thoughts.

EDIT: while I was typing this wall of text you responded seg with what I had exactly in mind :goodjob:

If not adding new NPCs I can use existing NPCs that would otherwise have nothing to do with qusts, just one of the Background NPCs.

I am sure you can figure out. And even if you use non-vanilla NPCs, like I said, I DOUBT anyone would know, so long as you don't give them anything that makes stand out like other NPCs. You know, just give your NPC common clothing, no visible weapon or anything.

Unless this particular Murderer is a sloppy one of course. There is just one huge brick wall standing in my way, I can't Script and I have never made a quest. Guess its tiem to start reading!

it has been awhile since I scripted a quest, but I found a lot of help (so long as you have patience) in the Construction set forums, and opening up other mods scrips to see how they scripted.
You need a small basic understanding of scripting...then once you have that, open up other mods, or pre-existing scripts and read how they work, then make a new script using that same script but transforming it to work in the way you want it to work.

That would be awesome, something rare for a change.

I have a limited knowledge in C++ (VERY LIMITED lol)

I'm pretty certain CS scripting is different from that...I mean, I haven't done any other type of scripting, so I may be wrong, but I don't think in real scripting there are scripts like:

"begin onEquip"

I am sure some scripting migh thave a similar premise, but still the CS scripting is a little different, I do believe

I know it is, TES Scripting is simplified script.

I think this idea is really good.
Theres lots of npcs that are not part of quests and dont do anything, you could use them. Then noone could tell who the murderer is because it could be anyone. Also you could use those npcs houses as part of the quests and hide clues an stuff. That would be better then adding loads of new npcs.