Want a man to take my place and fuck my wife (and me)

I had an idea for a fantasy recently, where my wife and I open up our relationship to another man, and gradually my wife and I stop fucking each other and only fuck him. My wife watches and gets off as he fucks me and makes me cum without even touching my cock, then he fucks her good and fills her up with his sperm after she cums on his cock. And it's the best sex either of us have ever had.