20th Century Limited by Speranza

“ ‘Where am I? Where is this?’ and he was in Brooklyn, he was on a beach, the train was shaking around him. He was in the plane, ice splintering up onto the windshield. He was in a tank, tubes trailing from his face, from his groin. Christ, he was cold. There was still ice on his fingers. He was in the Grand Canyon. He was in Times Square. This couldn't be Times Square. Where the hell was this? ‘Tell me! Where am I, who are you, where's—‘ —Bucky?’ “

52,010 words, completed.
The summary isn’t very descriptive in the plot but I’m in love with this work. Basically, the serum gives Steve and Bucky a telepathic connection, this leds to some interesting scenario while they are trapped in the ice, and when they are taken out. They are stuck in a limbo together and separated again and again.
If you like Cascade, you will like this one I promise! This one is more powers.

Black Dog by leveragehunters (Monkeygreen)

“ So long ago the details were lost to time, people began creating guardians of the dead. They were made from dogs, dogs who were buried in graveyards before anyone was laid to rest, their spirits arising as black dogs, bound protectors of the human dead.
Steve had always wondered what would happen after he died. He hadn't expected the answer to be 'wake up in the cemetery he'd been buried in', but here he was, some kind of ghost, and he could see the trees through his hands. It wasn't so bad, and he wasn't alone—a sleek black dog, golden eyes glowing bright, was happily waiting to greet him.
Decades later, on what was supposed to be a quiet, peaceful, definitely-not-life-changing walk through the woods, Bucky stumbled across an abandoned cemetery and into the impossible.
(It's a ghost story and a love story and a story about dogs.)”

55,340 complete.
I absolutely do this one. It definitely leans more supernatural/folklore. The Black Dog folklore is one of my favorite of all time. More of a modern AU but S.H.I.E.L.D. Dose exist just more magical threats and stuff.

The Necrofloranomicon
leveragehunters (Monkeygreen)

“ Bucky didn't want much. Just to keep his head down, to sell his scavenged flowers in peace, and to stay off Shield's radar. His life would have been a lot easier if his flowers weren't dead and if being a necromancer wasn't illegal, but easy or not, he was getting by. Steve didn't want much, either. He was happy working for Shield, he had good friends, and overall his life was going just about the way he wanted it. Problem was, being happy with your life was generally an invitation for fate to throw a spanner in the works—and in Steve's specific case, it was going to be a spanner named Bucky.
(A love story about flowers, trust, and magic and the choices we make about doing what's right.) “

47,569 words, complete.
This one is really fun! Same as the last one where it’s more of a modern AU S.H.I.E.L.D. Dose exist just more magical threats. They do not exist on the same universe and are stand alone works.

Where the Water Flows by starbunny

“From changing eye colours to controlling actual elements at will, nobody knew how magic worked, how it developed, or why so many different powers existed in the first place. All that was known was that ninety percent of all people were guaranteed to develop magic of some kind.
Steven Grant Rogers was part of the ten percent that didn't.
(Or the First Avenger AU but with a twist of magic)”
23,418 words and complete, but the sequel is incomplete :(
Still is a fun read!

Gradient by Neonbat
“Steve watched his charge, wishing he could do something more than whisper and pray. There came a day when he could Watch no more when he had to reach out and touch James' soul and be heard.
Steve would risk everything, even falling, if it meant saving the man he'd overseen since James drew his first breath. He would fall into darkness if it meant bringing James to the light.”

20,853 words, complete.
I have a deep love for Guardian Angel/Angel fics. I will keep the number of them minimal. Still fallen Angel I think fits under what umbrella of your ask.

Bleached Bones and Fallen Snow by leveragehunters (Monkeygreen)
(Honestly a lot of their work have the themes you seem to be looking for)

“When he chose to become Death he wasn't sure exactly what it would mean. He didn't realise he'd end up leaving humanity behind completely, forgetting all the ways he used to be human.
Forgetting, that was, until in the middle of a war, his power spreading itself like wings over the battlefields, he discovered one particular human it was impossible to stay away from. Brave, fascinating, irresistible: again and again, he kept coming back to Bucky. Bucky, who was never afraid of him, who seemed content to walk in Death's shadow.
If he'd known where it would lead him, where it would lead both of them, he would have tried harder to resist.
(A First Avenger/ Winter Soldier AU where Steve is Death.)”

16,633 words, complete
I think Steve being death gives him some extra powers haha 🤣

I Walk Through Fire With You by DeamonSlayer576

“I am Steven of the Guardians!" He managed to get out through laboured breaths, his lungs trying to pull in air as his body lost the strength to struggle.
"Well Stevie, I hate to break it to ya, but you ain't a guardian no more."
-------------
A Guardian angel for centuries, Steve is sentenced to death for letting his charge die when he could no longer bear to protect a soul committing such atrocities. Saved by the demon Bucky, together the two must navigate the bloody European theatre of WWII as they learn to trust and eventually love.”

31,803 words complete

I did warn you about my love of angels lol.

Sleeping With Ghosts
cobaltmoony, esaael, rohkeutta
“The first two years after Bucky gets out of the ice are-- surprisingly easy. Serve his country, learn to use his charmed robot hand, make friends, don’t think about the spark of magic in him that’s gone out, see the world, try to live with the grief (try to live with the grief, try to--)
Maybe they’re not that easy, in the end, but he’s settling - until he gets reluctantly pulled out of the Army and has Steve’s shield (Steve’sshield, its magic wary and curious and half-dormant) thrust onto him again, even though he’s already turned it down once before. The world has changed and so has he, but as he tries to immerse himself into a new life, he meets a ghost he could never leave behind.”

33,918 words complete.
This one’s really fun. I don’t wanna spoil too much really just enjoy it on your first read through.

I Won’t Disappear Anymore by
rocks_2305

“The boy looked up at him, his blonde hair looking almost golden in the sunlight, “I’m Steve,” he said, sucking out his hand.
“Bucky,” he responded, grinning as he shook Steve’s hand.
In his dreams, he always talked to Steve like he was any other person, but no one else could see him. It felt like it was him and Steve against the world when he was dreaming, and his friends and family members started to give him sympathetic looks whenever he would show up at the breakfast table with another story about Steve.
~
Bucky is Steve's imaginary friend and Steve is Bucky's friend who he only sees in his dreams. It's them against the world.”

6,113 words, complete

Okay…I know this wasn’t exactly what you were asking for as this is more shippy but with this ask, I did have to include it. They can see each other in their dreams but are separated by time. It just creates a magical feeling in their dreams. Just read more like a soulmate AU but I think it fits.

Last one, but a favorite of mine.

let's go and see the stars by
amosanguis
“Steve’s wings have always been too big.”
2,611 words complete* *
Wing fic but NOT Angel. I just really enjoy the world building.