Marvel Ultimate Alliance 2. According to Shacknews, he'll be playing a character "significant to the game's storyline," though more details are unfortunately unavailable.
Lee is apparently quite excited to have his likeness finally put into a video game, saying, "I am thrilled to be making my first onscreen video game cameo." What we're excited about though is the possible release date of MUA2, revealed by multiple online retailers to be September 15. We've asked Activision for comment on the release date and will update this post as we hear more.
Update: Activision has confirmed to us that the September 15 release date for Marvel Ultimate Alliance 2 is correct.