I experiment by advertising on complementary searches in cases where the search volume is low and I have hit the ceiling.
In economics, the concept of complementary goods describes things that you buy together.
When you buy a car, you buy fuel. When you buy a printer, you buy ink. You buy computer hardware and then you buy computer software.
Cars and fuel are complementary goods, as are printers and ink, and hardware and software. You can’t use one without having the other.
I did this for a client who offers immigration services to Australia. Someone thinking about moving to another country will also think about where they’re going to work and live. They’ll Google for jobs, property prices, medical care etc.
These searches are complementary to immigration services, in the same way fuel is to a car.
A search for “plumbing jobs in sydney” is a clue that the searcher is thinking about emigrating. So is a search for “perth property prices”. Advertising on these searches is worth testing.
I’ve written a little more about this here if you’re interested: https://pete-bowen.com/how-do-you-get-more-leads-from-google-ads-when-you-ve-maxed-out-impression-share