While this does mean that the 16 new levels don’t have a specific theme to them per-se (other than the normal-looking house with a normal-looking lawn and normal-looking lawnmowers) you will still encounter the themed zombies from the other worlds. In addition, two zombies from the original Plants vs. Zombies game, the Balloon and Newspaper Zombie, will return in the Modern Day update.
Furthermore, three new plants are being added to the game: Shadow-shroom, Moonflower, and Nighthade. None of them require money to be unlocked, so don’t worry. The Shadow-shroom, once eaten by a zombie, will poison that zombie, who will then poison other zombies. Meanwhile, the Moonflower generates sun based on how many shadow plants are next to it. And of course, you have your relatively normal Nightshade, which simply attacks zombies.
However, do note that this is only part one of the update, and you will need a World Key to unlock the modern levels for free. Otherwise, you will have to pay for it, although World Keys are fairly easy to obtain by simply playing the game.
