Check out this Reddit thread 
The top comment…
I knew a guy who… bought a very highly specialized machine that cut perfect holes of all sizes in concrete for all sorts of situations… they did extremely well indeed.
Someone replies…
I know a guy with a very similar business who makes good money doing even less work… He takes X-rays of concrete… bought a specialized machine that scans / xrays the concrete and shows if there are any cables, pipes, etc…
In many jurisdictions or building regulations it is a requirement to scan before the core drilling so his business is always in demand.
Chris Koerner chimes in on X…
The higher the percentage of people that never knew this business existed = the higher probability that this business needs more competitors.
Lots more examples of such businesses in the Reddit thread to spark your own ideas 