I don't know how they'd make biters/walkers/lurkers/zombies more dangerous than they already are. They're really just a background threat, not really ever supposed to be the primary threat. The living are the ones that need to be feared; the dead are just background dangers.
I suppose, though, since season 3 established that walkers can "starve" if they don't eat enough (they just starve really slowly), then perhaps the walkers are capable of more. If they need to eat, then they're still alive, and if they're still alive, then maybe they can learn
. That'd be scary - a smart zombie.
The first episodes of the first season had a walker use a rock to break a glass door down, so they have retained enough rudimentary intelligence to use tools. If only the smartest ones have "lived" through the first year after the apocalypse, I can see the smart ones being a tougher threat to the survivors, who have honed their skills on the less intelligent ones.