A few thoughts:

I am not sure anything made yet from carbon will become a classic. Barring some technological breakthrough, carbon remains too fragile to sustain a long-term pre-owned market. Right now, where the oldest carbon frames are still pretty new, and likely to have only had 1 or 2 owners, you might trust somebody to sell you a used carbon bike.
But too many people have seen some kind of crazy exploded carbon bike picture to want to ride a carbon frame you found in a dumpster (like so many of us have with steel frames).

Someone mentioned,almost as a joke, that Surly bikes might go down as classics, but I think there's an argument to made for it.
-Their bikes are iconic, simple, and easy to picture.
-They're fairly revolutionary, in their own way. Each of their bikes has wide appeal but fills a niche nobody knew existed.
-They've been widely imitated.

As far as actually answering your question (modern alloy bike with a timeless feel)...
The first thought I had was 1990s Cannondale.

It's still got the classic silhouette of road bike, but the oversized aluminum tubing and seamless joints give it a very idiosyncratic modern look.