You need to find a niche for your add-on. For example, if you want to develop a mail merge add-on, you will have to overcome many popular mail merge application such as Yet another mail merge, or Mail Merge with Attachment. I think that you could not have a lot of users because when users search and navigate the marketplace, these tools will take the number one and two spots in the search result.
You should be a big fish in small pond.
For example, in the past, I wanted to build a QR Code add-on. Which kind of QR Code add-on I want to build? I researched via some tools such as Google Trends, Keyword, Google Adword, and Google Workspace marketplace. All those tools are free.
1. Using Google Trend