History shows us that for something to be considered a true popular classic it must, in addition to being simply good design, have been mass produced, ie, made in enough numbers that enough people remember it from their earlier years for it to have some personal resonance for a big section of society, or for a whole generation. And, critically, it must have been mass produced for enough of them to survive to feed the interest. The Mustang is a perfect example along with the VW Bug, 32 Henry (and for us Brits) the Austin Mini. The resurgence of interest in eighties BMX and more recently 80's and 90's Mountainbikes will continue I think. I'd be looking for something with some historical significance like an early Ritchey, MK1 or MK2 Stumpjumper, early Rockhopper.... Ti Zaskar...... Offroad Proflex... though none of these except the Rockhopper had the production numbers to really become a popular classic.
Trouble is, as an earlier poster said, the cantilever frame is a thing of beauty in itself so a straight tube bike will never have the same draw.
Gotta be a Dyno if you want a modern canti frame with good looks and rarity tho. Harley Veloglide? Electra Fink?