the Jewish (Yiddish?) Dybbuk, which exists to replace someone completely, making themselves into that person and that person either into a Dybbuk or simply disappearing - it's been a while since I last read about them.
Chants are amazing, and really help for immersion, but only if they're well done. If you're gonna make some slapdash poems I think it's better to handwave it or just do gibberish
I haven't read any fanfics on SH, but if I found any for fandoms I follow, I would read them. On a wider scale, I know I've seen some AU fics perform very well, and get amazing ratings and readership.
More female leads that aren't genderbent/in a romance story. There are precious few stories about women coming into their own, alone, and it'd be nice to see one. I'm drafting one right now, actually.