I always like the thought of summoning a being like a demon or genie, and instead of wishing for something selfish like money, he just wants to get to know the being.
So my character would talk with them and get to understand what is going on. And he then would wish she wore diapers, and she can have what fun she likes including getting him into diapers as well. I don’t sub generally, but I can play a switch type setting.
Given this idea, the demon would turn a guest room into a nursery and want to decorate part of his room or something, but not completely take it over.
But since his desires are not selfish it’s not a soul trade deal. We can do that if you want to make it that way. I’m not super keen on soul selling.
Lastly, we can do NSFW, I imagine that way is more fun in general