Well I think a good way to evolve the zombies with time is the decay method, but with some differences. You see my friends, as the body decays, the flesh of the fingers and teeth become more recessed and eventually look more and more like claws and fangs. This contributed to the whole vampire scam of the dark ages as people were payed money to stab a dead guy who only showed common decomp, but I digress. What I am saying is to make it so that they keep the slowing effect, but as they decompose, bite and scratch with more damage to a reasonable point. The decomposition of their flesh should also work towards attracting nearby zombies as well, similar to how their own noises attract them. So in my conclusion, they would get more fragile and slow with time, but they would increase in horde and group density, and would be significantly more likely to attack with bites or scratch first thing rather than punch, and with greater effect. One could also increase their intelligence to a point over time, similar to older classic Romero zombies who would have then try to go shopping or drive a Mercedes to their homes in life, or salute crazy bloodthirsty captains in underground bunkers.