People are familiar with Influenza and the government would expect a seasonal flu. Influenza is an RNA Virus. It affects the respiratory system. Relatively low Mortality (a few percent). Rabies is also an RNA virus. But, it has an almost 100% Deathrate if not treated with antibodies. In the past few years, there have been several cases of rabies that have been reported with no history of a bite. It is an airborn rabies. It has been proven as fact and reproduced in the lab under controlled conditions.
Proposal: Influenza cross hybridizes with the rabies virus. We now have a highly communicable disease from the influenza with high mortality from the rabies. As an added bonus, rabies is “known” for its rage. These can be the runners.
I will take a while to think about the best way to get shamblers. If we have a second viral outbreak independent of the influenza/rabies hybrid that could do it. It has always been there, (similar to Max Brooks Solanum virus). It only makes shamblers. Normally well contained. But, if a few people become infected with both, things spiral out of control.
Just throwing out an ideas
I had envisioned individual only, but you bring up something interesting. It might be possible to have mastermind zombies. Take the tact that although pretty disfunctional, put a bunch of them together and they might be able to group think. Use the "nanite" concept. One does a single task, as more come together the tasks can be more complicated.
I have not seen anything in the "cannon" that leans either way. I would pass on the group think, because there is so much going on. Maybe this is something that can be explored in a lost zombies 2.0.