Backstory: There are so many review sites out there. Trust Pilot seems to be the most popular with ecomm sites but it lacks specific categories such as ship time and value.
The Idea: Ecommerce based review site and plugin.
Deep Dive: You would build a review site platform similar to trust pilot but cater specifically to ecommerce sites.
Validation: Clutch, https://clutch.co/, is a perfect example of coming after existing review sites and niching down. Clutch is exclusively for B2B companies like marketing agencies and dev shops.
Monetization: Similar to proof, fomo, shopify embed, etc... you could have a freemium version that sponsors your review site when an ecommerce company lists their reviews and star ratings on their site.
Steps to Launch: I would start by reviewing different ecommerce companies and getting traffic to your site. Then pitch the idea to those companies and offer to link their review ratings on each article. Those companies would probably be willing to switch or start a review platform given some free traffic.