You could try describing how the anus is a centre of multiple nerve endings and a source of great pleasure which may be interesting to experience, maybe she would too? A prostate orgasm is meant to be hugely satisfying for a man, so maybe after giving her multiple orgasms you could ask if she would like to try do the same for you.
Fisting must be one of the most intimate,intense and extreme acts with some of the most powerful orgasms experienced, hopefully she would love to see if she can turn you into a quivering lump of jelly.
If you can find some tasteful clips (especially homemade) to show her, she may find that fisting although extreme is a 'normal' activity enjoyed by many and that she might want to try it. My wife gets very wet when fisting me, and when she's sitting on her magic wand has more orgasms than I do... ;D
There are a number of resources in fistforum.com which might help or be of interest to you.
Good luck :)