I also haven't seen anyone write a fanfiction on Danmachi where the MC is of a different race like Elf or Demi Human so this could also become a good idea.
You could make it so the MC was born in the world of danmachi and not reincarnated or you could just make the MC die in his world then he wakes up in the world of Danmachi, something along these lines.
If you chose a demi-human focus on the abilities of the race, for example dog/wolf people would have more strength and better senses while cat people would be more flexible and faster, while elfs would have high intelligence and have a lot of mana or cool magic.
You could start the story around the time of canon or a year or two before canon starts.
Obviously you want the MC to be an adventurer so he has to be in Orario whether he was born there or traveled there from s different village.
It would be cool if the MC would be a free lancer, meaning he's not in a familia so he works for anyone that pays him or just goes into the dungeon solo. (If you decide to do this make sure to give the MC a system or the ability to get stronger without a Falna!)
Write about the MCs relationships with the other adventurers and gods.
You could also make the MC dislike the Freya familia or something like that.
The MC could find an other person of his kind and train him, make a party.
You could have Hernes and Hestia hire the MC to help rescue Bell and his party when they went down to the 18th floor. (This is when the MC would build a good relationship with the Hestia Familia and have their backing.)
MC could join the Hestis familia and help them during their War Game.
(A/N: Write about the begining of his adventurer days and how he trained. Relationship with the main characters of the story. You could also make him take a quest outside the city to protect a small village that was being overrun by monsters. Have fun!)