Ranked #1 In Google. What Is The Next Plan

So I have this client for which I have ranked already to the top position. Now he requests to target the next niche possible. It's a micro-niche site.

What would you do after you rank your site in 1st position for your major keywords for the niche targeted?

Would you move to the next shoulder niche targeting or leave it with minor updates occasionally to be safe from google?

What is the smartest move according to you?

If you're trying to make money off of them:

1. Go for spots 2-4 with other sites. It's not like #1 gets every click.
2. Push the need to diversify w/ more domains.
3. Recommend other languages (if possible), like adding Spanish for US sites.