Backstory: Often times people create a startup but never finish it. There is a wasteland of forgotten and "almost finished" startup businesses.
The Idea: A platform to sell your startup for equity.
Deep Dive: You would build a platform for someone to post information about their startup, why they started it and why they gave up. Users would also be able to set equity terms.
Other users would be able to search these startups and find ones they're interested in completing. The platform would provide the contract for the startup transfer.
Validation: There are already sites like flippa designed to sell businesses for money.
Monetization: You would either charge a fee to post or work a deal to take a small amount of equity for every deal brokered.