First thing is, is this a business or a hobby? Getting enough people to pay the facility rent etc is pretty easy, getting enough to replace one or more full-time jobs is a different and harder thing.
Second is, quality of coaching. Most people won't travel more than ten minutes or 2km to a gym, because most people don't want and can't get decent coaching there from the 19yo AIF graduate. But if there is good coaching, people will travel for it, I am in Clayton in Melbourne and have had people from Geelong and Mornington, Markos at PTC in Carrum Downs has had many more and from much further. Obviously most of your people will be within half an hour's drive, but if you can offer good coaching then some more will come from further.
Lastly, and this ties into the first two, decide if you want to be a traditional black iron gym (dirty toilets, rusty barbells, no aircon, Pantera playing) or something fancier (toilets cleaned 3 times a day, place vacced every day, good equipment well-maintained, etc). The first gets you the hardcore lifters, who are mostly broke and/or stingy, but hey your gym gets to be known for setting records; the second gets you 40yo accountants and stay-at-home parents, no records set but you earn money.
Lots of other stuff but some things to consider.