Yeah, they've amped the zombies up quite a bit. In the early Res, they were slow lumbering creatures. Then we got too good at killing them, so they put Nemesis in. Then we got good at killing him, so they sped up all the zombies and gave them ladders. We're just too good at killing. My flatmates say they are terrified of me when I play Res4: Mercenaries, all fall before my wrath.
If we go back to traditional zombies, then we need true realism as well. More like Condemned than CoD, as well. Start us off in an organic city, randomly, and begin the virus spreading. In the early stages, obviously we will have an advantage over the NPC's because the game title will have 'Zombie' in it so during that time you try and collect weapons, ammo, friends, start up a safe place etc.
As the number of zombies increases exponentially, the game will get harder and harder with a decent learning curve. Perhaps after a certain time, when you are expected to have either a decent base and/or weapons, the army can come in to try and kill everything, Half Life style. I'm not sure, but I think something like that is the only way trad zoms will return. From the dead.