Here's my idea for a series ender (may contain spoilers, so consider yourself warned):
I would have the last year of the show be season long arc where the characters finally discover the cause of the zombie virus and how the entire plague got stared. As a result of this discovery, they also learn that the virus is no longer active in the environment. It has died out. In addition, they discover the virus is not transferred from parent to offspring. What all that means is that Judith and all other children born after the outbreak are not infected. A normal world can be re-built by the next generation to come.
I feel the series would have to end on this type of hopeful note to be satisfying. The final scene of the final episode could involve a dying Rick looking at his daughter and smiling, realizing that there is indeed hope for better days for her.
HOWEVER, I could also see the following epilogue inserted after that scene: We cut to another group of survivors the show has been dealing with during this final season...a more nefarious group. This group comes upon the last remaining unused stock pile of the Zombie Virus. One member of the group with more conscious that the rest moves to destroy (incinerate perhaps?) the vials. Their leader stops him and picks up one of the vials, contemplating it in the light. "No, no now." He/she says, "Let's not be hasty. There may be use for this someday..." Fade out. The end....?
What do you think?