Backstory: I've been interviewing people lately for an open position and I sent someone a few bullet points of feedback and they responded with a long "thank you" email. I probed a bit and found that interviewers were not always giving feedback or even a response. Often times I don't have much time to give a detailed response of why I didn't hire someone but I would probably be up for clicking a few buttons ranking their skills.
The Idea: A survey to send to interviewers if you didn't get the job.
Deep Dive: You would build a simple survey website that contains a bunch of default questions for interviewees to send to their interviewers when they are rejected from the job. You can niche down into specific markets and have pre-built templates with appropriate questions such as "rate my technical skills" for a developer position. One thing I really like about this is having your branding on all of the survey's sent. Once you get one customer, you're more than likely about to get 2-5 more users viewing your site.
Validation: Survey Monkey and TypeForm have done quite well in this space.
Monetization: You would be able to charge for a custom template. You could offer a free version and sell one-off custom templates for a fixed fee. You could also partner with job boards and recruiter sites to send target ads.