Immigration is another one of those topics that lends itself to a cause and effect essay. You might consider the impact of immigrants (either legal or illegal) in an entire country, one region, or one city. Or you might consider the impact of immigrants on an industry, such as farming.
Hint: You could also take another approach. Discuss what would happen to the economy if immigrants were no longer allowed to enter the country. Or conversely, you could discuss what would happen if current immigrants left the country.