The best gym concept by far for you is to think about a budget gym model. Look at what they do and then see if the location is suitable. The idea is that they require very little work to run versus a specialist / full scale operation.
I really like the idea of a gym you walk into with a swipe card, minimal staff, low low cost to both consumer and operator...
And I own gyms but not like these ones. I wish I did because I think it would make for an easier life and a more scaleable business.