There's a suggestion - mostly propagated by Kevin Feige's biggest fans - that the grand plan for the MCU was very precisely plotted as far back as the early days of Iron Man's development. It might be a nice bit of mythology that helps push the magic of the MCU, but it's almost certainly a very precise exaggeration of the truth.
As the abandoned projects, changed plots and leaked, unused concepts prove, Marvel's flagship franchise has gone through significant stages of evolution, and what we got in the end could have been completely different. And even beyond the ideas that didn't make it off the concept boards, there were some that were actually seeded in movies but then led absolutely nowhere.
Sometimes, Marvel Studios either wilfully drop the ball (or change their mind and retcon things out of existence), or they just forget. With that many creators involved, these things can happen, no matter how careless they might look...
Having seen his nefarious plot to destroy Tony Stark and usurp his position as the world's premier weapons dealer, Justin Hammer is carted off by the FBI. But not before he says "I'll get my revenge on you, someday!" to Pepper Potts. Chilling.
What it Teased
When it comes to MCU villains, the general rule seems to be that they can return if they're not killed off very, very definitely. Even when they're sucked off into another dimension by an Infinity Stone, it's not final. So the fact that Hammer was left alive and vowed revenge seemed to suggest he'd be back.
Aside for mentions of his tech (particularly in the Netflix Marvel shows), Hammer has remained locked away with the key thrown away. It seems his parting words to Pepper genuinely were just empty bluster.