I got an idea the other day, and I don't have the time to give it the attention and treatment it deserves, so I'm putting my outline/concept for it up here in case anybody else wants to work with it:
The concept is a young woman, college age, who has never been fully potty trained, she wears pull-ups every day and has every day of her life, by choice.
When she was a toddler, she was very resistant to being potty trained. Her parents struggled with her over it, at length. Finally, they made a bargain, if she would at least use the potty for her #2, they wouldn't make any pressure on her to give up her pull-ups before she wanted to.
She made the deal. Time went on, she never, ever wanted to give up her pull-ups. She was good about using the toilet for #2, but she made zero effort to hold in her #1 and was totally used to being diapered.
Her parents would ask her if she'd like to try to wear panties, but she kept refusing, she had come to like being diapered. When she got school age, her parents homeschooled her so they wouldn't have to deal with explaining to public schools that their daughter had refused to be fully potty trained.
A decade and a half later, she's college age. Still in pull-up style diapers, and by this point her childhood attachment to them has become an adult fetish. She loves the feeling of wetting, the feelings of being in a wet diaper.
She's never had a dry night in her life. To deal with wetting the bed, maybe she wears heavier, bulkier diapers at night, either disposables or cloth.
So, in college she's had trouble dating. Most guys don't want a girl who wears a diaper (strange but true, and I like my stories to be very plausible), and even though she's very attractive and very intelligent, being padded has cost her a lot of guys when she goes on a date and they eventually learn her secret. This has taken an emotional toll on her.
She may have tried to toilet-train herself and wear panties with a pad in them to make herself more appealing, but found that even then she keeps having accidents and wetting the bed, like if she wasn't toilet trained at her age she never can be.
. . .until she meets that special guy who loves a girl who loves diapers. It's been a secret fetish of his, one he hasn't been able to indulge or explore, but now he's got a girlfriend who lives it, and it's his dream. He loves that she is padded at all times. He'd be happy to change her, or put her in more diaper-like diapers. He'd like to try wearing diapers with her.
Maybe a story of their romance, of him learning to like diapers too, of them being a happily padded couple, culminating in their wedding day with them both thickly diapered under her dress and his tux.