PopCap didn't release any information about the sequel, but has launched another PvZ title for Facebook called Plants vs. Zombies Adventures:
Unlike the original, Adventures has players defending a motor home from waves of zombies, and the undead don’t just walk in a straight line this time. They can follow paths that turn and twist, like in other tower-defense titles. New plants like the Beet — which, uh, beats enemies — will help deal with these more complex attack patterns.
Plants vs. Zombies Adventures also has a town-building mechanic that has players upgrading various properties to earn new resources. This aspect of the game also reveals PopCap’s social and monetization strategy.