Hi! I was wondering if you could rec some mystery fics?? Like there is a murderer on the loose or stuff like that?? A thriller mystery maybe even horror fics??? Thank you 💕

There are a few with police officer Steve and there are horror recs that you might like, though not a lot with real mysteries that I know of.  Here are some more suggestions:

DON’T LET YOUR LOVE BE A MYSTERY! THRILL YOUR AUTHORS WITH COMMENTS AND KUDOS!

Marvel–and the murder on Flag Display by MissBoaBoa:
An AU Marvel fanfic, starring Tony and Steve, two detectives who solve murder cases in New York.
Follow these two as try to find out who murdered Aldrich Killian, unraveling some dark and disturbing secrets along the way. Starring Steve, Tony, Natasha, Fury, Sam, Strange and a lot more of your favorite Marvel Characters!

The Mysterious Affair at Stark’s by FestiveFerret:

It was universally agreed upon that the best parties in Bannerly Park were those held at Stark Manor, and tonight would be no different… or would it?

Wild weather, loud noises, an argument, a mysterious stranger, and at the end of it all…

A murder.

See You Later by Tobezilla: In the small town, Ironside – a decade long mystery has
consumed Steve as he desperately tries to find clues that will lead him
to Peggy, his childhood best friend, who disappeared alongside her
sister, Sharon. It may take a set of new eyes to find the missing piece to this decade long mystery.

Gravitational Pull by antigav_vector:
A strange temple floating in space is discovered, and Steve and Tony are
the logical choices to go investigate. What they find is going to make
or break their relationship…

What We Could Become by xDinahQueenx:
When a close friend of his dies under suspicious circumstances, Steve
Rogers creates an alter-ego with which to investigate the  accident and
determine what really happened. What he discovers is more complicated
than he could have imagined.

Look here, look back, look ahead by marinarusalka:
September, 1941: Returning from a mission as Iron Man, Tony Stark
crash-lands in the Carpathian Mountains and is rescued by Captain
America.  The two heroes team up to investigate Nazi activity at a
mysterious castle.  But Captain America is keeping secrets that could
destroy their new partnership before it has a chance to begin.

The Devil You Know by @shetlandowl: Best selling author Tony Stark revives the bodice
ripper genre for a modern audience. From frisky gay cowboys to ravenous
lesbian pirate queens, he consistently delivers riveting thrillers full
of romance, drama, and the filthy, unapologetically kinky sex that has
become his trademark specialty. Tony has everything a man could
dream of – horny, adoring fans, and boatloads of money. Or that’s what
he thought, until Detective Steve Rogers walks into his life and turns
it all upside down.

Whatever Makes You Happy by @wilmakins: Steve and his team never stopped Avenging. So when
T’Challa tells them about strange events at the site of an ancient
landmark, everything seems quite simple. Of course they’ll help Shuri to
investigate it – helping people is what they do, after all.Until
they discover that a race of alien empaths are harvesting human
emotions as a source of power. Specifically, they’re using the misery of
one Tony Stark. So, until they work out what’s really going on, the fate of the world might depend on them keeping Tony happy.Not simple. Not simple at all. Set six months after CA:CW.

The Devil, You Know by @tisfan: When Justin Hammer is found dead at his home, Chloe Decker and Lucifer Morningstar are on the case…And who should just happen to be there, but Tony Stark and Steve Rogers…
Tony… who has a secret…

Comfort, Truth, Joy (the Mountains of Madness Remix) by @cptxrogers:
A mysterious signal is emanating from deep within Antarctica, and Steve
and Tony set out to investigate. If they can make it through the
freezing temperatures and the deadly ice fields, something even stranger
awaits them deep in the antarctic mountains.

Ain’t Nobody’s Business if I do by @copperbadge:
The year is 1930, Prohibition and the Depression are both in full swing,
and Chicago Police Detective Steve Rogers has his hands full. There’s a
dead body on the banks of Lake Michigan, the entire city’s legal system
is corrupt, and the king gangster of the North Side, Tony Stark, has
taken more than a passing interest in him.

Serve and Protect by gryvon:
Steve is sent to investigate a traitor at Stark Industries, Tony Stark
is way too young for Steve to be having improper thoughts about, and
everything is not fine.

there’s no dreams in the waves, only monsters by @s-hylor:
Driving while tired is never a good idea, especially when the weather is
terrible, as Tony finds out the hard way. Now in pain, with a wrecked
car and no phone service, Tony is stranded at a hotel perched on a cliff
overlooking the flooded ruins of an abandoned town in the valley below.
With no working phone, only three other people around, Tony isn’t sure
who to trust or what to believe. Something isn’t right, something is
stopping him from leaving.

An’ Harm Ye None by LinneaKou:
“…do as ye will.”
Toni Stark, small-town witch, has served as the head of the
Cravenswood town coven for a decade all the while assisting the police
in occult-related crime. Between her and Detective Steve Rogers, nothing
that happens in Cravenwood is too big to handle – not even the
reappearance of her estranged brother… that is, until someone starts
ritually murdering townspeople.As the death toll climbs and a
conservative religious group begins to cast blame upon the town witches,
something monstrous begins to stir up Craven’s Woods. Faced with an
unknown Craft and drawing the suspicions of the citizens she swore to
protect, Toni must put an end to the killings before whatever is
haunting the woods comes out into the light.Fear lives in a small town.

Wasted on the Dead and Dreaming by LinneaKou:
Steve looked right at his friend, and he felt just as disoriented
as he had when he woke up in Munich. “What the hell is going on?”
Bucky winced. I think you’ve spontaneously developed psychic powers. And well. Wasn’t that just great.When
Captain Steve Rogers got back from his tour of duty, he didn’t expect
to come back with the ability to communicate with the dead.When Tony Stark moved back into his childhood home, he didn’t expect his dead father to still be inhabiting it.When
Tony hired Steve to investigate the ghosts of his family mansion, they
didn’t expect to be swept up in cover-ups, corporate sabotage, and
murder.Now, time is running out as increasingly disturbing dreams
haunt Tony and the ghost of Howard Stark proves to be the least of
Steve’s problems. If they’re lucky, maybe they won’t end up ghosts too.

The Fire in Your Eyes by @veldeia:
Captain America is assigned the task of solving a mystery case of arson
with possible superhuman involvement. The billionaire adventurer Tony
Stark seems somehow related to the case, and Steve needs to figure out
whether he’s a victim, or perhaps the culprit—without getting too
distracted by his charm.

Man with Your Frame by @veldeia:
Steve Rogers spends his days as a body-swapping agent for SHIELD and
many of his nights exploring the various bodies in the bedroom with
young tech genius and business heir Tony Stark—until one fateful mission
changes everything. When Steve wakes up decades later, all his friends
are gone, except for a much changed Tony. He recruits Steve to
investigate a series of attempts on his life, but Steve soon realizes
that finding out who wants to kill Tony may not be the most important
mystery.

A Long, Lonely Time by asktheravens: Steve returned from the war injured in body and mind-
and able to see the dead. At loose ends and desperate to get out of New
York City, he accepts a fellowship through the Stark Foundation and
retreats to a quiet lake house on the grounds of the Stark Mansion. He’s
supposed to be there to paint, but he quickly realizes that the house
is more than he bargained for. Anthony Stark died here a decade ago, but
was it an accident? A suicide? Or a murder? Obadiah Stane still lives
in the main house just up the hill, and the past casts a long shadow.When
Tony’s ghost begins appearing to him, Steve becomes more entwined in
the dangerous mystery surrounding his death. Even worse, he finds
himself falling for a man who died a decade ago…Features lots of ghosts, murder, secrets, and supernatural revenge. Also Thor and Rhodey.

I Will Find You Through Worlds by @chibisquirt:
This isn’t Tony’s first adventure and it won’t be his last. But this
time he doesn’t have the faintest clue what he’s looking for, only it
certainly isn’t what he does end up finding: An unknown man in an ice
cube. Who is Steve Rogers, and why does he seem so familiar?

The Inside Man by Roysowrdsman:
Steve Rogers is a detective with Tony Stark as his partner. Though Tony
finds the missions they’re given a bore, he finally finds one that
excites him, and it’s a solo act. Tony puts his life on the line to save
his partner, but whether he’s pushed himself too far this time is
another thing. Can Tony really work solo on this one?

When We Were Just a Story by @winterstar95:
Tony and Steve are captured by unknown forces. They have to work
together to get out safely and Tony learns a little something along the
way, but has a ton of questions left unanswered.

The Mountain (Tony Stark Whump with Story) by EvilApril:
The Avengers investigate an old hidden Hydra base that has been off the
grid for years, only now resurfaced. Deep within an icy mountain they
uncover some horrible truths that should have remained hidden, and with
an unknown foe hunting them, time is against the team while they attempt
to unravel the mystery.

Missing Rings by GodofLaundryBaskets:
Tony Stark is throwing his farewell party before he goes off to the
European front line, when he introduced to one retired Captain Steve
Rogers. Steve enlists Tony’s help in finding a set of missing rings that
were stolen from the museum Steve works at, but not everyone wants the
rings found.

Skin of Your Teeth by tresa_cho:
Phil’s mostly recovered, but Clint’s struggles with himself aren’t over
yet. The Avengers are called together once more to help solve the
mystery of why people are disappearing all over the world.

(Stony is secondary pairing)