Darn, i would've love to play as human that accidentally falls through a portal to hell, is taken/sent to hell due to a crazy satanic cult, or possibly the human can us magic so can open portals to hell, but could have transformation magic so he can become a sinner, hellhound, imp, inuccubus, a goetia, and etc. Also the magic could help him sneak into heaven, where he could help Charlie out by sabotaging Adam and Exorcists or possibly expose Extermination Day to rest of heaven.
Cool and a future idea could involve gendering swapping some male characters, especially Moxxie for those don't wish to steal his wife. Here are examples of Female Moxxie Fansrt:
https://www.deviantart.com/chiizzz/art/Female-Moxxie-892712416
https://zatyrlucy.tumblr.com/post/668976124989227008/petitprincess1-cynicalwonde...
https://www.reddit.com/r/HelluvaBoss/comments/sjl58n/what_would_genderbent_moxxi...
https://www.reddit.com/r/HelluvaBoss/comments/wli9r0/female_moxxie_for_a_fanfic_...