I don't think C# would've been a good fit for flutter in its early days, but definitely Flutter should no longer be tied to a single language like Dart. I think you should definitely consider stronger cross-language support.
I'm not sure what that would look like. Flutter mostly written in Dart itself (the framework, tooling, etc). The Flutter Engine (C++) is probably less than 1/3rd of Flutter, but could certainly be made portable to other languages if that were useful, but I suspect said languages would just fork it or write their own.