Some random ideas I just came up with are below. Not all of them passive, but might still interest you.
- Website with WordPress templates for real estate agents.
- WordPress multisite network that enables real estate agents to create a site quickly and then pay a monthly hosting fee. You could scale this by selling to different real estate companies, getting all of their agents a personal site.
- Mobile marketing for restaurants. Create a service that enables restaurants to send text message deals to customers.
- Pick an audience that interests you and post user generated content. Then sell ads/sponsorships to companies that want to be in front of that audience. For example, if you create a site for people to submit cute dog pictures, a company like BarkBox might pay for sponsorship.
- Teach people a skill and use Amazon affiliate links to the tools that you use (if possible).
- Marketing analytics. Build a tool that integrates with Google analytics and provides recommended actions to take to improve website performance.
- Create a stock photo site for a specific niche (doctors, lawyers, start ups, etc).
- Create a directory for a specific niche and then sell top placement on that directory.
- Create a site that allows people who don't know how to code to make an app.
I used long codes (short codes are expensive!), and I didn't do the marketing myself. It's basically a way for them to keep a list of customers, enable automatic signups by texting the number, etc. They write their own messages and send them out.
I'm not much of a salesman, but I was able to make a few sales, mostly to local places via in-person conversations.
Anyway, I have this software built and I'm willing to talk about moving it into other niches.