It's easy to sit and fantasize about game x and game y, but you don't realize how much time and effort each and every one of our games take. Our team has immense experience with the cod engine, but it still took well over 3 months to finish and release WaW. It's not that we wouldn't want to support loads of games, but for every game we add, we also add significantly more overhead in terms of testing, bug fixing and other maintenance etc. Therefore it isn't feasible for us to "just add game x", especially right now since we're working hard on some very cool features for our current games.
As for BO3 specifically, it's extremely unlikely to ever happen seeing how popular it is on Steam still, and we would never risk the entire project for it.