The term classic is quite broad, It doesn't necessarily imply worth nor a specific year, but rather a vintage that is no longer produced. When I think of the term classic I think of a unique automobile that in some way set itself apart from the auto masses, or perhaps a vehicle that inspired others in the industry.
A couple examples:
Delorian - I agree with the previous posts, except that it is not saab underneath, but rather leftover volvo bits
Definately a classic with a cult following. Performance arguably mediocre and styling was/is love it or hate it. Much like the xf.
Saab 900 first gen - also known as a "C900" the "c" implies classic which actually ranged from 1986 to 1994. The last of the preGM models. While this car has not been around for too long most auto communities refer to it as a classic. I find this interesting as our xf's arguably have similar traits such as build quality, craftsmanship, and a certain uniqueness that only other owners understand. Is it a classic, definately, would it have been a wise investment opportunity...no way. My 94 was almost $40,000 new and it is one of the best examples around today and I would be hard pressed to get $10,000 for it,
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Chrysler Conquest/Starion STI
I thought I would include this also because I always thought there were many similarities between our modern day Chrysler. This car also didn't fair as well in the market as it probobly should have and today it is quite hard to find one in good condition. If you are lucky enough to find an unmolested example they are still very affordable.
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Now if your talking to a slightly different generation you could also argue the following are classics:
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1969 camaro, went from a 455 big block olds hurst drivetrain to a more managable built small block. (no more lightening sticks)
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1951 chevrolet coupe, big block olds 455, factory rear and front, drums all the way around still...scary
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Arguably a 1927 model a vintage hotrod built in the 50's ford flathead V* merc rear 4 on the floor.
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1937 chevrolet master deluxe mustang II fron 8in rear 455 buick big block.
All in all I would say the crossfire will likely "become" a classic at least in book. I would argue that its worth is not likely to increase (or stop declining) for at least 10 years...but who knows what we will be driving on the roads then.