Since the toxic-starfish were DCotE's most underrated 'scare', I personally would love seeing them return as vertabrae'd enforcers of some sort. -Intelligent, tentaclesque, suction-cups... weilding seahorse-shaped schimitars or something. Giant clams aren't exactly polite tea-party guests either.

One of the scariest episodes of Voyage To The Bottom Of The Sea involved the bringing-aboard of a mutant plankton-creature, which escaped into the submarine's air-vents. Guys open a door, and the room would be fine. Guys open another door, and layered green slime with suction-stalks would engulf them.