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I think both of the shorter greentext series (Vaggie fattening Charlie secretly as she's naive, Niffty going slob) could work fleshed out.
I still have an idea that hasn't been used yet though (namely, Charlie's hotel failing, and, in her state of failure, eventually deciding gluttony is a victimless sin, thus she can focus on that and still make her dad proud. She slowly goes from trying to impress to being a true glutton, then she goes full sloth, slob, and starts to sent ripple effects throughout hell, as the princess going full glutton slobby lazy prompts others to adopt that sin)