I agree with you that their probably is a large untapped demand for co-op games.
However i fail to see how going to this content method will help. Why bother to make massive changes to the way the game is designed, coded, compiled etc and then to release it to a lot of uncertainty when they could just throw together another horse patch knowing that they will make a good chunk of change from it.
More over if you look at the community to see what they have released first in helping to decide what sort of content you should release the sort of stuff they have lined up is what will work, hardly anyone makes a co-op mod and there are very few successfull ones out there.
Having said all that, I want Oblivion a mod to come out for oblivion to make it co-op.
Its not that im against co-op,i just dont believe that the dev's will attempt anything potentially risky.
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$1 tax for the national debt?
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Diablo & Diablo 2 for the DS, it makes sense Blizzard!