Proof that the remaining 10% is worth getting stuck down by a minotaur here:

General Fics

White Rabbit and Bashful Elf by DappledKarma

• Recommended by Markus Ramikin
• Status: Ongoing
• Synopsis: It started as a fine day for Bell Cranel. One meddling Loki later, a drunken Lefiya is spoiling for a fight. But don't worry, Loki doesn't escape the consequences, either.
• Comments: Starts as a cute and amusing take on Lefiya's relationship with Bell developing from a jealous rivalry into something more. While that is still in progress, a larger plot seems to loom on the horizon.

Heroes by The Four Crosses

• Recommended by 55Revolver
• Status: Ongoing
• Synopsis: A powerful figure has returned to Orario seeking Bell. What does this person want? What will happen to Bell and all those around him? How will the gods react, and, more importantly, what will they uncover upon their return?
• Comments: An interesting and exiting take on the Danmachi universe by adding a second World's Strongest Man to the setting to serve as a mentor, and father figure for Bell, and powerful ally to the Hestia Familia. My only real complaint is that the Author can't seem to decide whether or not Freya can or should be redeemable or not. But if you can get past the obviously overpowered OC, and the attempts at Gray-and-Grey Morality, it's an entertaining read that has interesting character development for several of your favorite characters.

Bell Cranel, The White Rabbit of the Freya Familia by Jyanzein (Spanish)/ Infinity 1412 (English translation))

• Recommended by Jlad Hyperion
• Status: Complete (English Translation Ongoing)
• Synopsis: After Bell arrived at Orario after the death of his grandfather, he cannot find a family to adopt him; not even Hestia accepts him. After reaching an alley, he goes to sleep without knowing that great surprises await him. Bell x Freya.
• Comments: This excellent fanfic delves into Alternate Universe in which For Want of a Nail, Freya takes in Bell and in doing so allows her to become more directly involved in his growth, leading to an interesting development between the two.

Is It Wrong To Try To Pick Up A Harem In A Dungeon? by Quoka

• Recommended by Fireminer
• Status: Abandoned
• Synopsis: Bell Cranel arrives in Orario, however when he is unable to find himself a Familia he takes a risk and enters the dungeon alone with no blessing to his name. Will he survive his trials? Will he find himself a Familia? Who really knows...only time shall tell. One thing is for certain though, he will be become quite the ladies man along with way. (Lemons included and clearly marked.)
• Comments: An excellent Bellx Syr/Freya fanfic in which Bell never met Hestia and join her Familia, instead living at the Hostess of Fertility inn. The story was originally on FF.net, but somehow it got deleted and the author posted a copy on AO 3.

The Infinity Elf's Return by Davis IceElf

• Recommended by KnightSpark
• Status: Ongoing
• Synopsis: Her world was destroyed. However Bell Cranel and the last remaining gods got together and sent her back to save everyone. So when you send an elf with lesbian tendencies who's in love with Bell Cranel to the past well... Let's just say that this version of Lefiya isn't quite normal. I mean, she'd pretty much and elf version of Loki who gets serious occasionally.
• Comments: With all of the crap this fandom has given Lefiya, I found it both liberating and refreshing to read a fic that decided to make her more badass (and a bit more perverted) than normal. Her interest in Bell, as well as developing her own growth, really gives the story a good zest to it. I don't keep track of all the times I have re-read it, but every time I do makes this even more enjoyable. Also, this is off the record, but Bete getting some comeuppance in the form of him getting Dragged into Drag was definitely worth a few laughs.

Is It Wrong To Worry About My Brother? by Born of Prayers

• Recommended by Leid
• Status: Ongoing
• Synopsis: As fate wove a tapestry to tell of a new hero for the age, so would the ancient vow be kept. Born of a human father and elven mother a year before the hero himself was a child in the Forest of Wishe. Though she would not bear the memories of her past, her heartfelt plea would transcend the ages. And through hardships and the Falna manifest, a new tale would be written in Orario…
• Comments: An AU story that primarily follows Lefiya Viridis, the reincarnation of the sister of Argonaut, reborn as the Half-Elf sister of Bell Cranel. The story follows both of their stories and incorporates plotlines and characters from Sword Oratorio and the mobile game while taking a look at various aspects of Elven culture, Magic, and character relationships. One of the longer Lefiya-centric stories at present.

Crossover Fics

Heroic Myth by haruoki

• Recommended by ThorIngerman
• Status: Ongoing
• Crossover with: Fate/Grand Order
• Synopsis: In the distant past, before the Gods descended onto the lower world, Heroes made contracts with Spirits to fight the monsters born from the Dungeon. But with the descent of the Gods and the advent of the Falna, those Heroes and Spirits have passed onto legend. However, with the discovery of a certain object in the Dungeon, these legends will return to life once more.

Grand Dungeon Oratoria by Wernher von Braun

• Crossover with: Fate/Grand Order
• Recommended by ultimomant
• Status: Dormant
• Synopsis: Some of the most famous stories are those that tell the stories of heroes of the past; people that managed to carve their own names through their deeds in the annals of history, immortalized long after their demise. In the present, once merely figures inside of records, the journey of a young boy shall begin their stories anew.
• Comments: The author admitted that this story was inspired by Heroic Myth, just with different Servants summoned, but hopes the story stands on its own merits.

Is It Wrong For A Godslayer To Have A Harem Of Goddesses? by Paulzies

• Crossover with Campione!
• Recommended by ultimomant
• Status: Dormant; last updated August 2020
• Synopsis: Kusanagi Godou just wanted to have the quiet, peaceful life he didn't manage to get in his past world. But who was he kidding? When there was a Dungeon full of monsters, beautiful goddesses, and animal-girls, there was no way that was going to happen. Still, the Gods here were way more friendly than the Gods he'd met in the past. Maybe just a little too friendly.

A Heroic Avenger by Nergal D

• Crossover with Fate/Grand Order
• Recommended by AftermathJack
• Status: Complete
• Synopsis: The Weakest Servant in existence, nothing more than garbage be it a Holy Grail War or any sort of fight. Yet upon gaining a new Master after a hero of justice stopped his corruption and sent to another realm, things just might change. In a land full of Gods and mortals alongside his new Master, maybe Angra can do something with his life. Hero? Villain? Avenger? Only time will tell.

Rabbit of the Moon by Born of Prayers

• Crossover with: Bloodborne
• Recommended by Winterkari
• Status: Ongoing
• Synopsis: In a different world, Bell Cranel died at the hands of the Minotaur on the Fifth Floor. The Moon Presence, searching for a new Hunter to bring the long night to an end, just so happened to stumble across his soul on the way to Heaven. Thus a contract was established.
• Comments: Our favourite rabbit flits between the worlds of Danmachi and Bloodborne, progressing the stories of both at the same time. The contrast works well. Bell feels earnest and pure in Yharnam compared to others around him. In Orario… well, Visceral Attacks. Enough said.

Incarnation of Legends by ObsidianNoir

• Crossover with Fate/Grand Order
• Recommended by Reppuzan
• Status: Ongoing:
• Synopsis: The Far East. Rakia. Altena. Bell Cranel, humble boy from the mountains, had visited these and many other countries in the world, inhabited by incarnations of legends. Those with great fates are drawn to each other, those with lesser fates led by them, to the center of the world. Will he be swallowed by the gravity of Orario or will he make his mark, to reach the Throne at the peak?
• Comments: Although the author takes some Broad Strokes with canon, the resulting story is original, thought-provoking, and all-around good fun. The butterflies of change are everywhere, and Bell's growth and lessons feel earned even before he sets foot in Orario after his little Road Trip Plot to get there.

Is It Wrong To Go To The Dungeon With My Wife? by Wernher von Braun

• Crossover with Fate/Grand Order
• Recommended by ultimomant
• Status: Ongoing
• Synopsis: To pick up girls in the Dungeon and form a harem. That is the dream of Bell Cranel, thanks to the tales that his beloved grandfather fondly told him. It is for that reason that he travels to Orario in hope of fulfilling the dream. However, it proved to be more challenging than he thought. After all, how do you fulfill it when you suddenly find yourself married?

Second Chance by Zae'lah

• Recommended by ultimomant
• Crossover with Black★Rock Shooter
• Status: Dormant
• Synopsis: When Mato Kuroi is about to die of old age, she manages to sever her connection to Black Rock Shooter so she won't die with her, then transports her Otherself to a new world, telling her to live her own life. Black Rock Shooter wakes up in Orario and ends up joining Hestia Familia.

Knives and Shields by Icura and Vahn

• Recommended by Dave 85
• Crossover with RWBY
• Status: last updated September 2021
• Synopsis: With the final battle against Salem concluded, Jaune now finds himself stranded in a strange world where there is no such thing as dust and actual gods roam the land. Caught in circumstances beyond his control, he struggles to find a path forward and perhaps a way back to Remnant.
• Comments: Jaune makes a sacrifice play to end Salem, and ends up in Orario. He settles into life with the one familia that would have a homeless bum with only a battered chestplate to his name - and naturally, it's Hestia's. He ends up playing the role of Big Brother Mentor to Bell both in the dungeon and without, figuring it's time to pay everything that was done for his sake forward. This ends up having a lot of knock-on effects.

Fate Grand Dungeon by Parcasious

• Recommended by ultimomant
• Crossover with Fate/stay night
• Status: Ongoing
• Synopsis: After the Holy Grail War, Shirou and Rin settled in the Clock Tower and Rin became Zelretch's apprentice. One day, Zelretch transports Shirou to Orario and promises that if he can conquer the dungeon and reach the bottom level, he'll find something that can help him reunite with Saber in Avalon. To assist him in this quest, Zelretch gives him a power boost so he can summon and maintain seven Servants. The Servants are all demigods, causing a lot of chaos and confusion when the gods sense their presence and relationships.

The Path That Glimmers of Freedom by Mundatorem

• Recommended by VictorioMWolff-Haddock
• Crossover with Fate/Apocrypha
• Status: Ongoing
• Synopsis: Sieg appears in Orario after he died. Mistaken as a new Guild staff due to his clothes, Misha drags him to and makes him work at the Guild.

Is it wrong to recruit new family members from other worlds? by Ubimarv

• Recommended by VictorioMWolff-Haddock
• Crossover with various franchises
• Status: Hiatus
• Synopsis: Bell was missing, the members of Takemikazuchi familia admitted that it was their fault since they led a massive horde of monsters toward Bell's group in order to flee. Hestia decided that she will form a rescue party of volunteers in order to save him...until a mysterious black-eyed man appeared in front of her with a deal.
  "I'll give you the finest warriors of many worlds"
  She didn't care if it was a pact with the devil, for Bell she'll even sell her soul. The Goddess accepted.
  The stranger left without stating the price...
• Comment: the Outsider provides Hestia with strong, otherworldly people who live a miserable life or died a painful death to save Bell and the others in the Dungeon, as long as she promises to be the Goddess of Hearth she is and provide them with a home and family they deserve.

God's Blessing on this Wonderful Dungeon by Special FX

• Recommended by VictorioMWolff-Haddock
• Crossover with: KonoSuba
• Status: Ongoing
• Synopsis: After Aqua presses a suspicious red button, Kazuma and his party of misfits find themselves in the world of Danmachi during the Dark Ages, eight years before the arrival of Bell.
• Comment: As a cover, the party pretends to be a Familia...with Eris, who got tagged along, as their goddess, due to Aqua losing much of her divinity, to her distraught. Since this is before canon, plenty of people aren't in their canonical Familia yet.