Backstory: I've noticed an influx of ladders.com emails in my inbox lately. Ladders.com is an over 100k salary job site, that got me thinking about other niche luxury markets such as vacation rentals (VRBO).
The Idea: A luxury vacation rental site.
Deep Dive: You would build a site similar to VRBO but present it as a luxury version. You could offer features such as concierge and higher resolution photos. Existing rental sites often have poor image quality and tend to have a generic look. You could explore custom templates and more modern designs.
Validation: Ladders.com is a good example of how you can enter an existing market (job marketplace) and succeed.
Monetization: Similar to other marketplace, you would charge the poster a percentage or set fee to post on the site. Having this serve the luxury community, you would be able to charge a premium.