An old thread, but how did you make out with it in the end, did you go for it?
There is potential there to do those things. The big problem is that you're in competition with every other landscaping company in town, most of whom do their own design work AND do the construction themselves afterwards. I've had experience working with subcontractors in Landscaping - contracting other companies to take over interlocking pavers - only to find that they turn around to do all of the work I do, and steal my customer in the process.
You're also in competition with landscape architects, who are known as the go-to for bigger design projects, and who have those networks in place and landscaping companies that they work with to do the actual construction.
You might approach landscaping companies as a freelance designer. It helps if you have some sort of credential behind you. In Canada a relatively simple one to get that still carries weight is the CLD (certified landscape designer) through Landscape Ontario and its sister organisation the CNLA. I'm pretty sure that they joined with PLANET in the states to make it recognisable across the continent. Some companies would be happy to contract that work out, it would be under their umbrella though, not the other way around.
The garden tourism idea is a cool one. They do that kind of thing in England, it's a big deal there. Check out the RHS and the National Trust there to get some ideas as to how to get that ball rolling in North America - I'm sure it would be possible. You might even contact travel agents - your prime demographic would be retirees, people who still use travel agencies.
Doing container planting is something I don't offer my client base, because most people are hopeless with plants and containers are a stressful environment for them from the start. The customer won't look after them properly (99% of the time), the plants will die, and you are the one that ends up looking like an idiot, because obviously, it's your fault because you planted them. I don't do it. Unless you want to offer regular care (at least once every couple of weeks) better to keep away.
Anyway, let us know how you made out!