Looking at how Brave Search is in early development, I’d suggest making it customizable early on, here are some of my suggestions:
1. Custom search indexing hierarchy: Social Media, Gov sites, Edu sites etc...
2. Ignore provided sites: "Hello, World! site!: github.com" Where the site '!' means NOT
3. Search via a regual expression
4. Only show sites with A11Y and or performance %: "Hello, World! min-a11y: 90" 90 being 90% via tools like lighthouse
5. Only show PWA sites: "Hello, World! strictpwa: true"
6. Only show sites that are "up" in the current region, using tools like https://www.isitdownrightnow.com/