Ive thought about modding and/or level designing for CoD for some time myself. I have made over 20 maps in the Unreal Tournament community and modded some gametypes there too.
The reason I spent a hell of a lot of time modding in UT was because UT release their titles and then let their community take over. You will struggle to find any stock servers in UT land. Most servers are modded to some degree, with almost all servers running custom maps and mutators.
The reason I have not spent any time modding in CoD is because of ranked servers. UT does not share that concept and so modded servers thrive, where as modded CoD servers seem very hard to populate because people care too much about ranking up. Ranked servers kill modding communities which is an incredible shame as it's modders who make stock games better imo.
If I ran a studio who were creating a new title that had ranking at it's core, a very high priority for me would be to find a way to allow mods to work with stock ranks.
So for example, the scoring system would have to remain untouched by the mod for it to be eligable to effect a players normal rank. I am sure it's not that simple, but you get where im coming from I hope
It seems these days that game studios do everything they can to screw over the modders, when deep down they know it's the modders who prolong the shelf life of any game. I dare say that if the powers that be weren't hell bent on releasing a new CoD title every damn year, the modders might get a bit of extra thought thrown their way?!