We need OS for mobile phones which don’t track us because Google (and others) gets most data about us via mobile phones. There also isn’t any alternative to Google’s android.
My idea is that Brave OS would be based on open-source android (without Google).
You should create Brave App Store, which would be a) store with apps checked for privacy (and security) or b) special tool to access Google play store it will protect you from Google’s spying on you (but I’m not sure if it would be 100% legal). You should also add “PWA maker” tool which would convert websites into apps. You will also be possible to download apps as .apk files.
Special “brave shields” app would keep an eye on the applications to prevent them spying on you and permissions for apps would be very strict (e.g they won’t be able to see serial numbers of your phone, they will have to use system file explorer to open/save files etc.)
Brave OS would lack some functionality, especially lack of apps (such as HUAWEI’s harmony OS), but I think that’s small price for privacy.