I do wonder if default actor isolation is conceptually different enough from the other three items on this list that it might benefit from a different approach to enablement. Strict concurrency level, strict memory safety, and warning control are (if I ignore a lot of nuance) generally speaking all ways of controlling diagnostics, whereas default actor isolation also has a significant runtime behavior component.
If I consider default actor isolation on its own without considering how the other three items should work, one idea which comes to mind is that it could be handled similarly to default literal types. For example, I can write this code today:
private typealias IntegerLiteralType = Int16
let x = 1
print(type(of: x)) // Int16
And something like the following seems at least somewhat aligned from a syntax perspective:
private typealias DefaultActorIsolationType = MainActor
And aligns nicely with the proposed SwiftPM usage of MainActor.self in Add a `defaultIsolation` static method on `SwiftSetting`. by hborla · Pull Request #8301 · swiftlang/swift-package-manager · GitHub