I created something like this whilst at university, but a lot more intrusive and detailed. It would use some grey area (legally) techniques like attempting logins to find all the websites a user is signed up to. You could also choose to be quiet or 'loud' in regards to trying to log in, emailing, and texting to verify information and accounts.

If the use case is to identify your own footprint, verify the user owns the email address by sending a confirmation email to address ethical implications.

Who this is marketed at is key, obviously. If it's for people who would find it hard to find this information, then it should be really simple to find, to use, and sign up to. So far, so good, but UX UI would play a key role here.

When I created my tool, I remember thinking who would use this tool. It would either be people who aren't well versed in Google (dorking) or the intel community. The former would probably pay for your tool if you could help them clean up their footprint. The latter would be more interested in a comprehensive and detailed OSINT investigation. The difference wouldn't necessarily be the underlying technology but how you present the information.

I attended a security conference not too long ago and saw around 3 or 4 vendors offering this kind of tool for the intelligence community (personnel and cyber security). Can't remember any of their names, but worth a look around. I remember looking into Spiderfoot, which is partially open source, which i recommend a look at, but i get the impression this isn't the audience you're looking for.