Backstory: Some sites have gone through redesigns with negative user experience effects. I've noticed this with news sites, they have narrowed the reading column, causing a lot of unnecessary scrolling on desktops. Additionally some have cluttered their articles with other content promotions to the point it makes the original article impossible to read.
The Idea: A crowdsourced dom manipulation to make pages more readable.
Deep Dive: This would be a browser plugin for tech savvy individuals to help cleanup sites that are littered with promotions and non user friendly experiences. Users could submit changes per site and the community could vote on different changes to come to a consensus on the best visible version of the site.
Non tech savvy users could still use the end products for sites they commonly use.
You could also have a personalization feature for users to tailor the site to fit them best. Personalized features could also lend itself to visually impaired users.
Validation: There are a ton of ad block and similar plugins out there already that manipulate websites.
Monetization: You may be able to charge a small yearly fee for users to use the plugin but you could offer it for free for life to anyone that submits a site improvement. The more valuable angle here is to provide a better experience for visually impaired users.