If you could add just one more hero to the Filmation line-up it would naturally be Metamorpho: The Element Man. Well, believe it or not, a Metamorpho series was not only planned (as were series for Plastic Man and Wonder Woman), the dude actually got a pilot made - though no one has ever seen it. Here is some proof.
Here is a note from Bob Haney which appeared in a 70's era issue of Brave and the Bold which claims an adventure was put on film and shelved.
In the '70s era Steel #3 in the Extra Daily Planet section in the back's "Answer Man" column, once again a 1968 animated Element Man pilot was confirmed. In fact, its been rumored that as many as six episodes were completed before being shelved. The reason for the series shelving has been blamed on Filmation's The Adventures of Batman. As production on the Batman series ramped up, the limited animation staff was reassigned to make sure Batman arrived on time.
Makes you wonder if Metamorpho may have made it big if he had gotten this break. As it is, most people outside of committed comic book fans have probably never heard of the guy. At least the Element Man did finally get animated on both the Justice League and Brave and the Bold series years later. So what gives? Will we ever see the footage? or is it lost forever to history.