Rare at least in my book has always been reserved for classic cars, I had a 68-1/2 Mustang CobraJet...that was rare, I had a real 68-Z28 with Cowl Plenum air cleaner and "in the trunk" Jardine headers w/Protect-O-Plate...that was really rare, 63-1/2 427/4spd Galaxie...pretty rare. I have a 14 Mustang TrakPak...one of only 4,777 made in 14...is that rare...No! Is it desirable...I wanted one.
I just can't see any new car being rare.....desirable yes. My 03 Terminator with now almost 6000 miles if maintained exactly in this condition..would it be rare....no I don't think so even in 20 years...would it be desirable...I'd venture a guess and say yes to some collector maybe.
The 08 GT500 I am trying to buy was one almost 15,000 made in 08 and I don't think it's rare............it seems to be desirable though.