Excited to see this step in the Flutter future. My biggest issue with using Flutter was deep dependence on Material or Cupertino widgets. In a way, it's cool to have all those widgets out of the box, but as soon as you want to create your own design system, it's a pain. Especially given, that Material/Cupertino is overly focused on mobile experience, and Flutter nowadays often used for desktop and web apps (with wasm being soon default renderer <3). There are only few well-maintained custom design systems for Flutter, and that's a shame.
This documents explains well the history of the choices made at the beginning and the current vision of the Flutter team.