I took the original question posted as a 'specific person'; maybe an old boyfriend, ex-husband, or perhaps a friend who has always expressed an interest.
My original thought was I should probably pick a celebrity of some kind (Sam Elliott, or one of her other favorites). While it would be fun to see, of course, it just ain't very likely. So I think of someone who has (or did) 'trip her trigger' that may be more realistic.
Now, when I also consider the fact that I'm a pervert, I'd like to see her fuck one of two guys.
One was a former live-in boyfriend (years before I met her). I've never met the guy, and know nothing about him, other than what she has told me over the years, and it all boils down to he was a good fuck and had a nice dick. They apparently were pretty intense together, but eventually broke up after a few years.
The other was a guy she dated in high school, and had a random meeting with (totally by chance), while she was on vacation in Baltimore some years back.
Supposedly, they hooked up after that meeting and had a play and fuck fest that had quite the affect on both of them.
If I think of in terms of a general fantasy, like some others have written, I always wanted to share her with military guys.
She's always had a 'man in uniform' fetish, especially if they are the young ones. I've watched for myself when she will approach them to talk or ask questions, mostly when traveling through airports. She will squirm while she makes up her mind whether or not she will do it, but gives in once she realizes she can't resist, and always comes back with names and addresses she can write to. Occasionally, phone numbers are shared.
In any case, I always fantasized about her welcoming 2-3 of "her boys" when they would have leave for a weekend or longer.
She could prepare home-made meals for them, take a shower or bathe with them, and remind them what good pussy and blow jobs, enthusiastically given, can do for a man.