When Worlds Collide
Logan's Run
The novel of the latter has an epic scale to it, one that I'd love to see translated to the big screen (rather than confined to a couple of cheesy domes outside of a junglefied Washington, DC). The former is the great-granddaddy of Armageddon and Deep Impact, but is so hopelessly dated (effects- and science-wise) that it almost screams to be remade.