I've been running a campaign and the party REALLY likes murder mysteries. They've been taking so many side quest that involve murder that I'm running out of ideas. If you have any cool ideas please post them! (There classes are Bard, Fighter, Druid, And Draconic Sorcerer all third level)

Have a villain (hopefully from earlier in the campaign) send out a call under an alias. Make it look like its going to be a classic murder mystery (mansion, plenty of guests/servants as suspects) and then have it be an ambush to prevent the characters from fouling up their plans elsewhere.

I have an assassins guild operating in my homebrew world.  They could have a brush with the guild when someone they think well of is murdered.  Then they don't just have a mystery on their hands, they have the whole guild to deal with.

Have you done identity crises like being extremely convincingly framed or even unknowingly compelled to murder?

How willing are the players to do what the supposedly good guy tells them to do with little question? Have you tried to convince them to do a quest to find and apprehend/kill an innocent person (an innocent person who was maybe framed or compelled to murder as mentioned above)? Maybe the good guy was trying to make the players take the fall for the murder of the innocent person as well as tie up a loose end.

I tend to like the ones where the players find themselves as the villains through deception of some kind. trying to prove one's own actual innocence when one is caught red-handed in the crime is no trivial task.

You could have a bandit ambush the players on a road. Later, it is revealed that an important nobleman was murdered, and the PCs are ordered/try to investigate. It is revealed that the bandit was the nobleman, who was desperate for money after gambling and piling up huge debts. The PCs were the killers all along.

Quote from Thauraeln_The_Bold >>

You could have a bandit ambush the players on a road. Later, it is revealed that an important nobleman was murdered, and the PCs are ordered/try to investigate. It is revealed that the bandit was the nobleman, who was desperate for money after gambling and piling up huge debts. The PCs were the killers all along.

Consider this idea looted! I love the twist! now to work out how to drag out the investigation to keep them second guessing...

pretend that its not a mystery and hint them with clues that lead them to doing the main quest

Would need to know more about their background and the games they have run through so far.

For example take a group of 4 adventurers a tiefling guide whose a swashbuckler about to become a Hexblade of the Raven Queen, a gnome Transmuter whose a teacher at the City Mage Academy, a half elven female arsonist whose the apprentice of a City Magus and an Outlander Cleric who stays on the outskirts of the city as they work as a Herbalist.

During their short career they have fought bandits, a fiend and confronted a trio of Orcs and some undead in the form of some Wiil-o-wisps and the zombie servants of the Fiend.

The Tiefling has family so he receives a note supposedly from his sister, but whilst he has suspicions its might be fake its an invite to a disused Mansion a little along the coast from the Port City they're based at.

He invites the others to what is apparently an evening meal and arriving early the weather is getting clearly worse and upon entering they find themselves locked inside as the entire building is now within the Shadowfell their only means to escape is to solve the riddle inside the mansion to return to their own world or survive until dawn and hope it reverts everything back to normal.

All they know about the place is that the owner of the Mansion is a reclusive noble with links to the Gnome's Academy and the Half Elf's Master.

They're initially attacked by undead from outside the building bursting in, forcing the cleric to turn them. But they won't stay away for long so they need to search the Mansion find out whats really going on and solve their predicament.

The twist is a message from the Tiefling's sister indicating the message was genuine, but it appears she now wants him out of the way. Possibly because she has found out about his career change that he has kept hidden even from his comrades although the Cleric has already figured that out and is only worried about whom his Pact was with.

Is that the sort of thing you're talking about or specific mysteries?

Any mysteries are fine! 

There is and Elf Bard, Elf Sorcerer, Goliath Fighter and A Halfling Druid. There all at fourth level. 

The Elf is looking for their long lost brother while the Sorcerer is looking to protect people from raiders. The fighter is in it to try new foods (wants to become a chef) and the Druid left their caravan for adventure.

Knowing the backgrounds they went with would help immensely but off the top of my head.

The Sorceror has some unknown link with the raiders so at some point they learn the Raiders are looking for something or someone eventually leading to some call back to the Sorceror's bloodline or family perhaps their mentor.

The Goliath's interest in the culinary arts might be shared by his family and he might be the latest member of the family travelling to develop their own culinary specialty?

Maybe the missing brother is someone the druid is familiar with and has their own reasons to want to find them?

I'd reveal an old nemesis of the Sorceror's mentor is behind the Raiders using them to secure specific plunder whilst they might get most a portion is given to them so they have their help.

Could make it an Order of Sorceror's and eventually they are going to run into members of that Order who are sent out to deal with the problems they've been posing to their operations?

Eventually they strike at their mentor directly and perhaps reveal the Elf's brother is that nemesis and that Bard encounter's the children of their brother who are mostly part of that order leading to them trying to turn them away from that dark path as the Druid reveals that brother was responsible for a series of attacks on the halfling's village or druid's grove leading to their wanderlust?

EDIT: Oh hold on what kind of Draconic Bloodline?

If your Sorceror is say a Red Dragon you could have the nemesis be a Gold Dragon but where the Sorceror I assume is maybe good aligned the nemesis isn't?!

Probably missed by a mile!

Thanks! This helps alot!