I watched this recently for the first time. It was good--amusingly droll yet sweetly earnest. It also struck me that the zombocalypse genre (and similar "last survivors" narratives) is largely about wish-fulfillment fantasy, reflecting actually a contemptuous desire to see the world and humanity go down in flames--a sentiment that Zombieland seems self-aware of, although it lacks the scope to explore just how short-sighted such a dream would be.
At the end of the movie, the male lead, played by Jesse Eisenberg, more or less acknowledges that his life is a lot better post-zombies, because the struggle to survive has brought him together with strangers who then become the family he has always longed for. It's probably a nice bonus that anybody he might have disliked is now dead. Basically, the zombies thinned out the human race to such an extent as to either remove all the people who previously were too good for the loner Eisenberg, or else brought them low enough to finally associate with him. He even wins the heart of Emma Stone, which the movie tells us would have been unthinkable in the pre-zombies world. Frankly, I find the pairing of these two characters (not the actors) unlikely even under such end-of-the-world circumstances.
Even supposing they could be mommy and daddy of this family, with uncle Woody Harrelson and little sister Abigail Breslin, I wonder how long such a group could hold together in harmony. At the center of this arrangement, every other character defined by their relationship to him, Eisenberg, a worthless loser in his former life, is now the luckiest guy on the planet. Emma Stone should have it pretty good too. And Harrelson, who already lost his son to the zombies, can no longer expect his life to get any better than this. But what about Breslin's character? She's happy now because she's a kid. But what happens when she gets older and hormonal, and the only guys in her life are the broken (and way old) Harrelson and the taken (by her sister) Eisenberg? Her life at least would suck at that point, and personally I don't see that there's any answer to that question that wouldn't lead to the dissolution of the whole family.
I suppose if there ever were a sequel starring a teenage Abigail Breslin, they would probably just have the group meet other survivors, including conveniently a young man her own age (or, I dunno, 15 years older or so).