Interesting topic.
My dream guns (2) are both FN F.C.S 25's that actually found 'ME' a few years ago. Both are based on the Fabrique Nationale Model 13 "Speciale Chasse" which was the precursor to, and the FN-equivalent of the BrowninG Superlight with straight grips and Schnabel forearm profile and weighing 7 lbs (with 70cm bbls).
Both were engraved by Sophie Purgal (one at the Custom Shop circa 1985-86 for a special FN project (paying tribute to 6 different countries) in the engraving design of her choosing which was German Mythology - the other is a 'custom' engraving done in her studio at home in 1996 for a client in Holland.
By my definition, the best Dream Guns uniquely represent a sense of individualism. I like surprises, myself - i.e., the ones I never knew to exist or those that enlighten us of what is possible. I didn't go looking for any of these and probably wouldn't have been able to justify the cost had they shown-up with an American money-grabbing, fantasy-price gun pimp. But when they appeared to me via special friendships, and I had to establish a value with no prior sales history - I had to trust my instincts. The examples you find at a reasonable value can also enhance 'the dream' part of the equation.
I think certain guns find those who are most passionate about their details and history, especially when they appear in a most-unlikely manner, at the most opportune time, and definitely striking a passionate nerve. After all, how many shooters would find 'boxing hares' appealing? Lucky for me, I guess. The gun represents the bond of special friendships fostered during its appearance, purchase, 4-year restoration, and appreciation of functional art by its admirers.