The roadster is one gorgeous machine, and I'd happily succumb to a mid-life crisis and buy one if I could, but I think it, and the forfour were the victims of MB being overly ambitious for the smart, which is why they're both gone.
If sales of the 451 help bring the smart division solidly into the black, we may see MB looking to -slowly- increase their lineup again. But I don't think we'll see quick reintroductions of the roadster, forfour, crosstown SUV or crossblade anytime soon.