Marvel's Avengers has a tendency of teasing characters that have yet to appear in the game. Its methods for doing this involve dialogue that players can overhear as they enter rooms or walk past other characters, as well as the many audio logs that chronicle the behind-the-scenes events that took place before and during the game's story. One of these even teases a potential appearance from Iron Man's longtime rival Justin Hammer.
Hammer, like Stark, is a genius businessman who owns his own company, though he's not quite as successful. As a result, Hammer tries to shame Stark and try to take over his company. In the comics, he even made Tony's suit malfunction so that it kills a diplomat in Iron Man #124, inadvertently leading Tony to spiral further into alcoholism. Hammer's appearance in Iron Man 2 shows off his talent for outfitting villains like Whiplash with the latest technology to take on the armored avenger.
In Marvel's Avengers, Hammer doesn't seem to be actively opposing Tony, though audio logs titled "Regarding Justin Hammer" sets up a potential villainous appearance. It chronicle Hammer's rise to power and eventual manipulation by AIM and Tony Stark. The first features Hammer gloating about how he took over Stark's legacy and is rewriting it in his name. The short letter is incredibly petty but fitting for a character as cocky as Hammer. It also alludes to big plans for the company, though he quickly learns that he isn't in control in the following log.
The next two audio logs feature AIM Scientist Supreme Monica Rappaccini sternly telling Hammer that he only makes weapons for AIM. Furthermore, if he tries to usurp her authority, she will remove him from his position because she owns controlling stock in the company. Following the meeting, a deleted audio log sees Hammer realizing he's in over his head with no way out. Thankfully, he doesn't remain in this position for long, as Tony extends an olive branch to his rival -- though that could also mark a moment of humiliation for Hammer that turns him into a greater enemy.
Following Hammer's acceptance of Tony's assistance, it's revealed that Tony was only acting like he needed Hammer to get some of his tech to reverse engineer against AIM. Once the new tech is complete, Hammer is no longer needed. While it's clever on Tony's part, Hammer will undoubtedly figure out what happened and plan his revenge -- which sets the stage for an appearance. Interestingly, Hammer is not historically a fighter, meaning he'll employ even more villains for the Avengers to face.
For now, there's no confirmation that Justin Hammer will make a full appearance in Marvel's Avengers. However, small events like the Red Room Takeover might allow for Hammer to show up and mounts an offensive against the Avengers without being a major campaign villain. The audio logs paint a clear picture of who Justin Hammer is. With the same pettiness as his comic and film version, Hammer could make Stark wish he hadn't double-crossed him.