Since you either didn't read the article you posted, or didn't care enough to share the other possibilities, I'll just repost this from your link:
The paper also does not explain the third studio's role, saying only that it will be "working on future versions" of Call of Duty games. Those may include the current console and PC mainstays of the series, handheld spin-offs, Wii ports, or even possibly mobile games like the just-released iPhone title Call of Duty: Zombies. Another possibility is a rumored massively multiplayer Call of Duty spin-off, which cash-flush CEO Bobby Kotick teased in 2008.