It sounds like a boring ending, too.
Here are a couple ideas, which aren't particularly great, but might help turn a few gears in your head.
1. A cure for the plague is discovered, and a character injects himself with it and sacrifices himself to the zombies in order to communicate it to them.
2. A cure is discovered, and can be distributed easily, but it gets to them too late, and the scientist character looks down at his army friend character, a zombie looking back up at him. Possibly a single survivor the army friend and his fellows had died to protect.
3. I often think about a minor subplot in some online chapter or two I read, about the start of a zombie plague. The character was in his fortified house, with electricity, reading an internet post from a woman several states away, trapped in her attic. She's not heard from again, and the subplot isn't pursued, and I'd've liked to have seen it completed.
4. Little is frequently made of loved ones in zombie worlds. How would someone react if they came face to face with their now zombie wife, if she and they had been fully realized characters and not just plot points? If the movie I Am Legend had ended in him seeing his wife and son in the attacking mob, would he have shot them, or joined them, or been prevented from (or encouraged into) one or the other by the new lady or boy?
5. What radio or television transmission might the survivors hear or see before or during the final battle that might be significant to them?
6. What if the zombiedom is an evolutionary step toward a higher form of life? Another idea along the "maybe we don't have the right to shoot them" line. I'm not terribly fond of it, but, again, wheels turning.
7. What if the zombie outbreak was a deliberate action by some sort of evil doctor character? This alone is, of course, the most common approach, but what if there were actually a reason to be sympathetic toward the evil doctor? What would compel you to view him as the greatest victim of the situation, if, having been exposed to him throughout the book as a fleshed out character, you saw him doing something or watching something or thinking something in the closing scene of the book? I don't mean in an I Am Legend the movie sense, I mean he actually set out to turn everyone into zombies, for some reason. Maybe it was necessary?