Tomorrow's classics are what the 15-25 year olds want to ride today - but can't quite afford.
Fat bikes, 29'ers, Steamrollers and (why not) Dutch bikes.

Narrow tired road bikes are iffy for future classics, I think. Proprietary parts will really put the hurt on some things. Eight speed Dura Ace is bad enough, but what are the odds of finding a replacement sending unit for a first gen electronic shifter in 10 years let alone 30. If you thought Swiss threaded bottom brackets were a challenge ...