Backstory: Restaurant reviews can vary widely from yelp to google and facebook. It would be great to see all reviews in one place and an overall score that is relevant for the past six months.
The Idea: A restaurant review aggregator website.
Deep Dive: Not all sites will have API's, you may have to build a scraper which always has the potential of violating TOS. You could get mechanical terks to update review averages based on date.
Validation: I've seen a few Quora posts asking for this exact idea and none of the responses were a true solution. Most were for business owners to manager their reviews cross platforms.
Monetization: Since all of these other services are free, you would have to follow that same model and monetize via advertisements.