I know this thread hasn't been used for about a month now, but I also was interested in this murder mystery pack idea, although I was too inhibited to comment originally. Though with the new pack trailer due for release in two days (Oct 20), I decided to just add my thoughts, which build on your original murder mystery pack idea.
I realise many people have said that a murder mystery pack is unlikely to be released because of murder not fitting with the teen rating in TS4. But my idea is a little similar to Dannydanbo's idea of a Murder Mystery Masquerade party, but it is held inside a hotel. So Hotel's would be the Expansion pack (which is something people voted as the number one EP theme on DigitalSpy). Part of that pack would be the opportunity to hold themed weekend parties, like the Murder Mystery Party, or Halloween and other seasonal parties, or even weddings.
Since the Murder Mystery theme weekend would be a party and not a real murder, it wouldn't affect the teen rating of the game.
Also, the Hotel EP would create new careers that could include progressing from starting positions as cleaning staff, a valet, or even a concierge attendant. Since cars are not currently in the game they could be introduced so the valet could park the guest's vehicles or at least be seen to drive the car away. This could have negative and positive outcomes, such as crashing the guest's vehicle or being tipped and eventually promoted for taking extra care of the guest's vehicle. The career could progress to managing the hotel and eventually becoming the owner. (Just as an aside, those guests that don't have cars could arrive in a taxi.)
Additionally, because there would be themed parties and weddings, this could also incorporate careers as party/wedding organisers, wedding registrants.
There could also be the opportunity to include the PI career like AuroraBoreallis mentioned, for investigating cheating partners or thefts (which would also bring burglars back into the game). Plus the Halloween themed party night could also create the career of a paranormal investigator or a psychic and also bring back Bonhilda as a supernatural hotel maid.
The Hotel theme could also be used to create a new Create-A-Destination-World, so people could design their own destination world (i.e.: Japan, Paris, The Moon, etc) and place their hotel in that world.
So the Build and Buy items could include various landscaping and cultural themed items to create the destination world, for example, Japanese wagashi walls and tatami mats, shrine garden ornaments and torii gates; or statues of the Eiffel Tower and Arc De Triomphe; paintings of the surface of the Moon or other planets; as well as some novelty Clue (Cluedo in the UK) themed paintings of the characters from the game like Miss Scarlett, Colonel Mustard etc., themed ornaments like the a candlestick, a revolver, maybe even a miniature version of the board game that could be placed in the reception of the hotel for Sim guests to play, as well as place in their own homes which could be used to build up their logic and creative skills.
New CAS could include Sherlock Holmes styled heads or a female version like Miss Marple, or Japanese Miss Sherlock, or a unique looking Lisbeth Salander type with piercings. PLus some outfits could include Sherlock Holmes styled PI hat, cape and tweed suits; Velma from Scooby-Doo styled clothing, a PI raincoat and hat in both black and brown colours and available for all genders; as well as a Miss Marple styled hat and outfit. There could even be a dress-up version of the clothing items for children to wear.
Anyway, that is just my idea and I doubt EA or anyone would like it, but since it would be an EP that could incorporate so many desired ideas (for instance hotels, destination worlds, cars, burglars and more), it would have the opportunity to create a lot of build and story play, especially when incorporated with other existing EP's, GP's and SP's.