Hentai, in it's very core, would be a metamorphosis but then the word metamorphed to mean perverse sexual desire. But that's only in Japan. Globally, especially in the United Stated, hentai is more of a catch-all term for any Japanese media with sexual acts in it. So if you want to experience it, in Japan go have some weird sexual fetish, like getting tuned on by having ducks in the bath tub or something. In America, get a copy of Dragon Pink and watch it in the basement.
Onii-chan does not advise erotic roleplay unless in the company of a person who wants to erotic roleplay with you. If you have that, pick a fantasy you both like and make it happen. Bonus points if you can get her to wear a school girl skirt and thigh highs and pretend to fall forward with toast in her mouth, revealing her striped panties.
I suppose you could try the upgrade meme, or is that dead already?