“I want to start a business, not a start-up. Which business has the lowest barrier to entry with the highest ROI?”
The highest possible ROI is usually related to shady deals in the gray area (or outright illegal ones). Due to the nature of those business ventures, ROI is massive simply because the stakes are high and you can easily get caught and do some jail time.
Therefore, I would strongly advise against similar activities.
With regard to honest business deals, that depends on your expertise and your target market. Here are a few ideas.
Think about eBay or Amazon. Or aircraft brokers. Or bookies.
Resellers can yield some decent ROI with negligible investment on their end. You don’t own any products or services. As a broker, agent, reseller, or marketplace owner, you’re the missing link between vendors and clients.
This could be applied in almost any capacity. As long as you can build the right network, it’s incredibly powerful.
Digital services (from software development through building websites or offering content/SEO services) require a computer and a stable Internet connection.
Sure, going upscale requires some investment in tools and business licenses. But that’s not a necessity once you start.
Digital services are often in demand worldwide. This expands your potential list of customers and can be scaled—a lot—based on your expertise and portfolio.
A professional could charge several hundred bucks an hour. Some can yield 4–5 figures an hour after spending a decade in the industry and building outstanding personal and business brands.
Once you start charging a premium, you can write your process and hire assistants or professionals who can handle most of the heavy lifting at a fraction of your fees.
Your Strong Suit
Most people forget to look at their own strengths.
For instance, native English speakers can teach Business English for large corporations with remote team members. Art majors can create online courses, speak at universities, resell artwork to prospects.
Plenty of niche businesses has been founded by owners who excel in a given industry and are passionate about educating the market. Online courses, product marketplaces, premium consulting or keynote talks can yield five figures a month or even more in a few years.