Treyarch and Infinity Ward. The two companies who alternatively develop the next Call of Duty games might have another studio to add to the development process. According to a LinkedIn post, Sledgehammer Games is looking for help in developing a "next-gen Call of Duty game."
This isn't Sledgehammer's first attempt at a CoD title, as they have helped co-develop 2011's Call of Duty: Modern Warfare 3 alongside Infinity Ward, however, that is the only game that they have worked on since then.
This is from the LinkedIn page.
Description:
The Sledgehammer character team is looking for a top notch hard surfaces modeler to join our AAA family working on a next-gen Call of Duty game. In this position candidates would live, eat and breathe everything vehicles and weapons related. They will work with the Lead Character Artist, Art Director and Design team to generate hyper realistic assets for in-game use.
Ideal candidates will have a passion for the subject matter and a willingness to dig deep into the world of military equipment. They should drive hard and challenge themselves with the goal of being masters of the craft.
Given that it's already January, I'd wager this isn't the Call of Duty title we'll be seeing in 2014, and since they stated 'next-gen' in the description, it's most likely not a handheld version either.
