Basically any company offering special services that work with very large data sets. That could be a consumer backup system like Carbonite or a bulk photo processing service. In either case, legal agreements with customers are key, because you ultimately don't control the storage system on which your business and their data depend.
I work for a non-profit doing digital preservation for a number of universities in the US. We store huge amounts of data in S3, Glacier and Wasabi, and provide services and workflows to help depositors comply with legal requirements, access controls, provable data integrity, archival best practices, etc.
There are some for-profits in this space as well. It's not a huge or highly profitable space, but I do think there are other business opportunities out there where organizations want to store geographically distributed copies of their data (for safety) and run that data through processing pipelines.
The trick, of course, is to identify which organizations have a similar set of needs and then build that. In our case, we've spent a lot of time working around data access costs, and there are some cases where we just can't avoid them. They can really be considerable when you're working with large data sets, and if you can solve the problem of data transfer costs from the get-go, you'll be way ahead of many existing services built on S3 and Glacier.