This is huge. Experienced lifters and bros talk all the time about “how great it’d be to open a gym” but overlook the fact the the overwhelming majority of people who will buy a gym membership are “average” not-super-intense-or-particularly-dedicated people.
A gym is a business, so treat it like that, not like a cool and easy way to make money with your hobby. As far as going niche and opening a powerlifting-specific gym or a strongman-only gym, unless you have a very strong network with established pros (competitors, meet organizers, etc.), it’s almost certainly not going to work because then you’re limiting yourself to a sub-group (strength athletes) of a sub-group (consistent, dedicated lifters) of a sub-group (people who buy gym memberships).
@Alpha recently had a video answering a bunch of business questions related to his gym. Doing anything the way Alpha does is probably a good idea.
Also take a look in the Personal Trainer’s thread. Some of the links and posts there should help point you in a direction.