I'll keep this short.

The company that always comes to my mind when I think "future classic" is Surly. Of course, for one, their bikes are steel—which we know is capable of holding up for tens of years (at least!). And the parts they use tend to be steel too—no plastic brifters on these bikes.
But perhaps more importantly, they are unique and idiosyncratic. Nobody makes bikes quite like Surly, and if they do, 9 times out of ten they are simply aping an established design. The LHT might be an exception, as I've always seen it as a tribute to the great 1980s tourers, but those are classics too.

Surly creates niches where there previously were none. This is the definition of future classic.