Honestly, it depends; sometimes I have a really good idea for an AU and I think "the characters from the OG media don't suit this as well as I hoped..." and in that case I usually end up tweaking the AU, ironing out some things and making my own story to go with it. Sometimes I have a great idea...
Try to tie it in with the environment. Does your dungeon have multiple sections? I'm guessing so since every ten floors is a boss. If for example the first ten floors were jungle-themed, you could have (sentient) man-eating plants, or brightly-coloured birds that spit poison. If it's a more...
Ah I meant if they became someone else's ghost (reincarnated into a dead person lol)
By Spirit I mean like... Not a conventional ghost? Does that make sense? Like they don't have a clear form and can barely talk or interact with the environment because of it.
I'm actually in the middle of writing this one;
essentially, a girl goes back in time to save the guy she likes from being banished for a crime he's committed. Every time she goes back, she ends up needing to go back earlier because there are so many factors involved in him committing the crime...