Character development related quests would be great. I think they were already well-developed, but there's probably room for more. Katie can have more encounters outside of the main story, and outside of dungeons
Quest ideas: A quest where you have to seduce someone to get information out of them. A quest where you have to pressure someone into giving you what they want, but their power takes you by surprise and they easily overpower you, but after the act you still manage to get what you wanted (info? an item? A key? A note?). A quest where someone is struggling to cum or get it hard, and you have to figure out how to get them hard.
The full shower of thoughts:
Crime mistery investigation (Thieves, people dissapearing), sex worker quest (can be done with imp, hotel, and maybe other places), delivering a package without knowing the contents that should contain something that the receiver might want to use on you, a quest made by a guild worker, counseling, helping someone pick out an outfit, witch disguised as elderly in need(they transformed) that wants to trap you, a ghost that needs your help with a favor (sexual or not), a few dates with the elf and Katie, a quest where someone asks you to spy on someone, a quest where you have to plant or protect nature. A quest that plays with the idea of drinking cum or dealing with a lot of cum, like searching for an item in a pool of cum(edit: There's something like that already). A bodyguard quest. Fake quests where someone like the slime girls tell you to go somewhere because it's important but it's just sex. A quest where you cure someone's injuries. A quest where two people are competing, and you are the one that has to choose who won (best dick, best breast, best pleasure, best retention, best lips, etc). Missing pet quest. A quest where you teach someone your ways of feminization and being more open (That's probably a job for Beatrice, but maybe they want your personal advice for some reason). A quest where you are the test subject for a device. An stealth infiltration quest. A quest where you have to help someone stretch their anus, probably without sex to reserve anal virginity.
I know they are adventurers and it's a guild, but it's still possible to make it so people don't take the guild too seriously and are willing to post quests that are less serious because they know they can get away with it. For later levels, since you are more experienced it would make more sense to have quests that presents themselves as more dangerous (even if they aren't), I think it would be good to revisit some areas to for example, get ingredients for Beatrice. Nothing grindy or tedious, maybe a small puzzle or a secret area, something that makes it feel different and doesn't feel tedious.
With pre-existing material, the church and Daisy is all I can think of. Maybe the elfs and Beatrice too?