While Ironman 2 may not be considered the best of the Ironman series so far, one thing you must agree on is how good Sam Rockwells performance as Justin Hammer was. Witty, cocky, and clever, he is literally the mirror image of what Tony Stark used to be like before his incarceration in Afghanistan which may be the main reason why there is such friction between the two.
Whilst he may not be you're typical Ironman villain Justin Hammer may actually have been someone who could have taken down ironman if not for his ego.
While most people including Tony Stark seemed to think the key to ironman style suits was actually the arc reactor, Hammer went ahead and built working prototypes without an arc reactor in sight.
Suits that were utilised by Ivan vanko aka whiplash as drones to take on ironman. An idea that was later used by Tony Stark to form the iron legion in age of ultron.
On returning to the role, Sam Rockwell told screencrush
I was hoping to come back in one of the ‘Avengers‘ films and come after Gwyneth Paltrow, because that’s what they set up, that I was going to get revenge on Gwyneth for getting me arrested or something. I could do a ‘Cape Fear’ thing with Gwyneth. But they’ve got a lot of mojo in there already, so I don’t know if they need another pirate like me in the mix there!
So what does the future hold for the character? Well you never know what with Tony Stark still refusing to help the government build their own suits (beside letting them use one of his old ones as warmachine) maybe there's room for Hammer to work for SHIELD in exchange for his freedom or maybe engineer some sort of revenge from prison.
In any case it would be a shame to let such a good character go to waste.