Thee's a couple of ways to stretch the show
1. Have one season by flashbacks of other survivors (so say, end of season 4 end with our heroes meeting one or two other groups and it looks like a standoff. Season 5 gives us the flashbacks of the "The Others", so we see what they are REALLY about (i.e. burnt out heroes like our own, or devious villains). Season 6 resumes the fight, and see who survives
2. Parallel stories around the world with, with a season where they all join together. That could make some interesting spin-off series, where say Korea (who are experts in arced dramas ) gives their own perspective.
As for "new" zombies...there could have been larger groups of survivors for a while that get decimated.
perhaps some wars happen whee many people die (and they don't get bodies for fear of being shot).
Maybe a plague happens, and many die before a cure is found.
Not everyone knows that ANY death makes one a zombie, so people find out too late.
Also, the winter might have been particularly cold, so decomposition process was literally frozen for a good long while (the opposite of Chicago's last winter).