Dwayne McDuffie's run had several of these, as he was infamously fired for speaking out against the ridiculous amount of Executive Meddling his run received. In addition to developing a friendship between Firestorm and Doctor Light, McDuffie had planned to have Icon join the team, as well as appearances from some of his other Milestone characters.
After clearing away the last remnants of McDuffie's team, James Robinson assembled a brand new Justice League line-up filled with characters like Mon-El, Starfire, Cyborg, the Guardian, The Atom, Hal Jordan, Doctor Light, and many more. Because of a variety of reasons, the vast majority of these characters had to be written out of the book after just one arc, leading to Robinson having to assemble another new team.
Back in The Bronze Age, Robert Kanigher set up a Love Triangle between Batman, Green Arrow and Black Canary. Mike Friedrich began his run the very next issue, and immediately got rid of the development by claiming Canary actually loved Batman like a brother, not in a romantic sense.
After Green Arrow killed Prometheus at the end of Justice League: Cry for Justice, it was mentioned that Green Lantern and the Flash were unable to find the advanced, computerized helmet Prometheus used to defeat his opponents. Astute readers remembered that prior to his death, Prometheus had essentially lobotomized his partner, Ira Quimby, who mysteriously vanished around the same time the helmet did. It was pretty clear DC was building things up to have Ira become the new Prometheus via a Clothes Make the Superman or Brain Uploading situation, but this never occurred.
Stargirl's backstory in the 2013 Justice League of America volume. In the first issue, it was stated that A.R.G.U.S. had been keeping her under close surveillance due to the unspecified fate of Sylvester Pemberton, the man who wielded the Cosmic Staff before her. After entering a burning building to rescue civilians, Stargirl discovered a glowing pentagram that'd been seared onto a wall, which led to the visibly scared heroine muttering Pemberton's name. As this was happening, Steve Trevor's narration mentioned that Stargirl always sleeps with her bedroom lights on, due to suffering from night terrors brought on by an unknown psychological trauma. All of these hints were seemingly meant to indicate that she was terrified of Pemberton, who'd been menacing her or had attacked her in the past. However, Geoff Johns left the series before this could be explored any further, and the new writer concocted an entirely different backstory for Courtney, one that firmly established Pemberton as a heroic figure who'd died quite some time ago. While Courtney's fear of the dark was seemingly explained by Shadow Thief having killed her little brother, neither the pentagram nor the suggestion that she was being stalked by Pemberton were ever brought up again.
