Completely new with a vanilla wife. How do i Star the subject

When you wake up in the morning, tell her that you had a wet dream about her being fucked by another man while you watched and you loved it.
She will probably tell you that you are mad, but I bet the next time you fuck her you will feel the different.

You just need to "plant the seed" and let it grow..
The planting seed is so true. I brought up mmf to her, lost her mind, fucked her later that night and she was gushing from having to think of it. It opened the door
 
More brought up open marriage to see what her interest level was. I’m still very into her, and she knows this. She just doesn’t have a high sex drive anymore
Just another note. People understandably can get very attached to people who meet their needs and we have a strong association between sex and being married or with that person we are having sex with…that’s why I think it would be hard for you to have this sexual relationship with someone else and get fulfillment there without complicating things at home. I am curious how she will react.