Huge swathes of the web (websites and web 'apps') are plagued by poor accessibility.
My suggestion is to build something that is not tied to a particular niche or industry, but almost everyone uses: email in the browser.
This would not be an actual working email service with a functioning back-end - it would be a front-end demonstration of how to implement a fast, accessible service.
Some things to aim for that will make it standout from existing email services:
- The email service when rendered in a browser will load instantly without complicated JavaScript tricks. Your service will be accessible to screensavers without the need to have two versions (a 'basic', limited version and a 'enhanced' version you can overstuff with JavaScript).
- The service is built with HTML with the correct semantic elements so that it can be navigated quickly by a screen-reader and by keyboard. JavaScript is perfectly fine, but the service should be functional even without JavaScript (e.g. progressive enhancement).
- The email service when rendered in the browser can be zoomed into at least 300% without breaking the display of elements and still remaining usable. There will be sufficient contrast.
- The site must load faster than any existing email service and be usable no matter what someone's connection speed (even a 2G connection).
- Finally, it must look aesthetically pleasing too.
This might all seem impossible, but with HTML + CSS (and JavaScript when needed) it's perfectly possible to create something lighter, faster, less complicated and more accessible than what the major players produce.
Here is an example of an email layout from the Yahoo! Pure CSS framework - this is actually quite old, but the HTML source is less than 10kb: https://purecss.io/layouts/email/