Some guidance

I registered recently but have been here a few months reading articles and enjoying this great site.
I am looking for some guidance with a recent development between my wife and I, where this fantasy is concerned.
Some background for you, we have been married for 20+ years. We are both in our late forties and our sex life has slowed down over the last few years.
I have had this fantasy which leans more towards the hot-wife side of things, I am not into the humiliation aspect of cuckolding at least not anymore then the humiliation that comes from another guy satisfying my wife in ways I can not. I have brought it up over the years and learned a few things along the way.
This brings me to a couple nights ago I woke her in the middle of the night horny since it had been a couple weeks since we went at it. When she asked me what had me worked up I told her I had a dirty dream. I asked her if she wanted to hear about it and she said yes. I started telling her about us being on vacation at a resort in a tropical location. We had arranged a couples massages which lead to sex between her and the masseurs. One a woman and the other a man from a photo her friend had sent her of a guy with a huge cock. When I was done with the story she had still not cum from me fingering her and I started asking her if she thought she could even handle a cock that sized she said yes then back pedaled a bit saying she thought she could. The she said she would like to find out and I whispered in her ear I would like to see that and that was when she had her orgasm.
It felt like a milestone to me it was the first time she seemed open about it to me. The last couple days have been sexually charged we have both been more playful and she let me finger her last night on the couch while we watched tv. Our youngest son was in hes room and normally she would never let me do anything like that with them home.
I was tempted to bring up a big cock again but I was unsure how far and how fast I should mention this fantasy. In the past she has cooled if I pushed something to hard and I do not want undo whatever it is that is happening now.
So from any of you with experience how would you proceed?