It wasn’t supposed to be Swamp Thing, of course; Swamp Thing isn’t a “supposed to be” type character.
After Superman was a huge hit, the next superhero character to line up for a feature film was supposed to be Batman. The character had been around for almost as long as Superman, and was easily the second most popular DC character. It was Superman and Batman headlining World’s Finest Comics, and the Super Friends cartoon, plus there was that TV show that everybody liked.
Everyone in the world knew that if DC was going to make any more superhero movies, then the next character in line was Batman — except, of course, for every movie executive at every studio in Hollywood.