Backstory: One of the slickest niche apps I've seen is stick albums. It's a photography website where clients can "download" their albums by making each page a link on their smart phone's home screen. The site uses local storage to store the images so you can access them anytime, anywhere.
Taking this same concept of a Progressive Web App (downloadable website) you could apply it to nail stylist.
The Idea: A platform for nail stylist to showcase and catalog their work.
Deep Dive: You would have to give customers the ability to sign up and link their account to their stylist. The stylist would be able to tag each photo with the specific client. The photo would be presented on the stylists gallery as well as on the customer's personal page.
The customer would be able to save their page to their home screen so they can see past nail styles. The page would also feature new styles so they can plan their next appointment.
As you gain traction you can look to build more clienteling features for the nail stylist like appointment booking.
Validation: Personalization is everywhere, customers would feel special knowing they have a history of their nail styles.
Monetization: You would charge a monthly fee for the stylist to use. You could go down the freemium route and offer a limited set of features for free and keep the good ones behind a paywall.