They actually had this in the form of super brawl during beta- several modes would spawn monsters into a lockdown-like arena. I'm not sure why they chose to ditch it so fully, except for the possible perceived necessity for class based combat as opposed to just not. It's also of interest that the playerbase isn't really large enough to make something like this work- try playing a game of blast network, and see if you can get in. A competitive game like this would stretch the lockdown community thin, though it could be just considered a subset of lockdown... i.e spawn people randomly and have class switching things in the middle or something, base it on general performance (like blast network) rather than capture points. Would still be considered "lockdown".