As spotted in a KGN interview with Randy Pitchford of Gearbox Interactive apparently has turned down, in recent history, a chance of doing a Call of Duty game. His explanation may be parsed out from the quote below:

"For me, there are two scenarios in which a project makes sense for Gearbox: First, if there would have been the game without us ever. Or two, when we can contribute to an existing brand something new, a kind of unique perspective and a new approach. But that was not the case here. I think about the people delivering the "Call of Duty", which they want, you have to play by the rules of the series. You have to do what is expected of a "Call of Duty". In this I see but no real challenge for us. It would strengthen our reputation as a studio no further, it would not be really motivating for our team."

Hidden not so subtly behind those words, we find the fact that Gearbox refused basically to do another cookie-cutter version of Call of Duty, and had decided they either wanted to do something absolutely of their own ideals, or not do it at all, as the former would definitely not have flown well with the corporate executives in Activision. As sad as it is that we won't be able to see a gearbox influenced Call of Duty, it is an admirable decision on their part, where their decision to refuse shows their strong sense of studio identity and spirit. Let's hope they go on instead to create even greater videogames than before, with this same determination to innovate that caused them to make this choice.