I haven't focused on diapers that much, since they're my third or fourth fetish interest. They're one of my oldest, though, from about as long ago as I became infatuated with enemas.
Nope, this isn't going to be about anything as messy as expelling an enema into diapers. My interest in both goes back to when I wasn't more than a few years too old for being diapered to be 'normal'. (OK, my enema love starts before I had the language skills to talk about it, even to ask for one.)
No, my best diaper fantasy is wedded to my passion to have an emotionally strong, smart woman spank me regularly, in the context of a love relationship. I want to be over her lap, naked (or at least bared to about the knees,) with a fluffy cotton adult-sized diaper as a barrier between her skin and mine, so I don't get pre-cum or the actual item on her legs or whatever she's wearing. There's the extra benefit that the sensation against my penis would take me right back to toddler stage, but with my adult erotic sense added on top. Mmmmmm!
I can also imagine it being really nice if, after she's finished spanking me to a decidedly red, hot-to-the-touch, stingy state, she rubs some soothing lotion over the tender places, then diapers me and makes me stay that way the rest of the day/night. Not for, you know, the usual functional diaper purpose, but just for the dominant/submissive infantilising humiliation jazz of it. So I'm reminded by my skin sensations who's who in our relationship dynamic. Only in guarded privacy, of course, 'cause we're both people who prize our public dignity.
I used to fantasize about my lady keeping me diapered from before I arrive until I leave, and when she takes my diaper off for play, it's a test of whether I'm being a good boy or a bad boy (whether I have an erection enclosed in the diaper from the excitement of her letting me know she's removing the latter.) She keeps her cards close to the vest, and changes the rule from one time to the next, but for one condition, I'm getting a spanking, and for the other, I'm getting an enema.
After either treatment, the age-play gets dropped long enough for me to show my enthusiastic appreciation of her loving care, by that most demonstrative of means of pleasure-giving, on my knees in front of her. This, after all, is not done between a charge and his nanny or a boy and his mommy, but between a rapturously submitting good boy and his beloved domme.
After she's been duly rewarded for being so good to me, she puts me back in diapers until they need to come off again. A decision that is always hers, naturally.