honestly, although it wouldn't satisfy me internally, I'd probably franchise one of the low-monthly payment gyms... franchise formulas work if you work them...
otherwise, a small, crossfit-style gym is usually popular.
JC Santana had a seminar on 1500 sq ft = $100,000/year take home...your gross revenue would be $400,000-$500,000. The idea was all about fun, fast, effective workouts, usually timed, group circuits.
You're money-maker is usually middle aged women with disposable income and it has to be fun/pleasant... rarely will you make a ton of money with a "hard core" gym full of dudes.
"You must have a very strong reason for doing an exercise. If you don't, scrap it and move on." -Jim Wendler