That’s a really great idea. At worst, you leave the company with a roadmap of what they need to do and that’s real value. At best, they can just hire you to do it :)

I’ve found that when you present this in a very objective way, companies are even more into it: “I’m going to leave you with a roadmap, not a sales pitch. You can use it to hire me, hire someone else, or do nothing - I’ll support you guys no matter what you do.”