It's somewhat old, so things might have changed by now.
Edit: Well, I did something. If you go to pvzf.fandom.com and then to User:TheNileGuacadile, you will see that most pages have no background image while my userpage does (although it might be some sort of error on my end).
I'm not really sure of the best way to come up with a world from nothing (I don't really focus on them at all) but the best thing to do at the moment would be to come up with a theme/concept. Perhaps something cold/wintery because of the time of year (for me at least, I live in the northern hemispere)?
WAIT, I got a idea, maybe a Giant Glacier that shrinks over time, at the start of the level is a timer, and the later the time gets the more the Glacier melts until its 0:00 when the Glacier is completed reduced to water tiles