I have so many, it's difficult to choose!
As a sub, I'd love to be kept diapered 24/7 by my girlfriend, humiliated and cuckolded by her. I'd be allowed to wear only diapers in the apartment, unless otherwise instructed by her. When at home I'd be her chaste diaper slave: I'd pleasure her, do the chores, etc. She would bring home lovers as she pleased, or I'd have to seek them out for her, having to explain to them in either case that my diaper wearing prevented me from satisfying her. One of my favorite fantasies is to be woken up by the sound of my girlfriend being pounded by her lover in the next room, while I'm left to make myself cum in my diaper. If she decides I've been good enough, then I'm allowed to watch - sometimes.
I'd pick out her clothes for her, and help her into her clothes every morning. She would give me little style hints, like "cute", "flirty", slutty, etc. The teasing would be incessant, making sure that I was always desperately hard for her, but only (rarely) allowed to cum in my diaper.
I have a weight gain fetish, too, and I want to be force fed in this position. It would add to my distress as my belly grew to eventually eclipse my cock, hopefully until I reach the point where I can no longer change my own diapers (or make love).
As a dom, I'd love to keep my girlfriend in diapers much to the opposite of my sub fantasy. She would be my diaper slut, and I'd fill her up whenever I pleased. Her diapers come off only for sex, and after I blow my load in her she gets taped right back up to make sure that her diaper catches it all. Her wardrobe would accentuate her diaper as I saw fit, often just peaking out from under her skirt.
I'd love to force feed her in this position. We're both rather skinny, so I imagine she'd be distressed by the change but unable to stop it. I'd want to watch her grow and fatten, from her current size small to at least a size large.