George Fan created Plants vs. Zombies 10 years ago. To celebrate that anniversary, he's been showing off his original design documents and sketches on Twitter (opens in new tab). As you can see from his notes, originally the enemies were supposed to be aliens rather than zombies—specifically the aliens from his previous game, Insaniquarium, which he was planning to make a sequel to. Once he drew his first zombie he changed his mind, and that original sketch bears a strong resemblance to the undead who eventually made it into the game.
It's only when I drew this zombie that the game truly became Plants vs. Zombies. Before this you were fighting the aliens from Insaniquarium pic.twitter.com/M7BTNfxIGOMay 5, 2019
Another element that wasn't in the original design were the sunflowers. Initially there were tubers who had to be watered and then uprooted to earn currency. The sunflowers, who produce energy on their own, seem like a definite improvement.
You can also see that at one point Plants vs. Zombies would have had six lanes rather than five (opens in new tab), and before that there were no lanes at all with more traditional tower defense mazing instead. Somewhat ahead of its time given how common it is now, there was even a deckbuilding mechanic.
For more on the history of Plants vs. Zombies, here's our story on How PopCap conquered casual gaming (opens in new tab).