ES6 will almost certainly not cover syntax for defining class variables. Only methods and getters/setters can be defined using the class syntax. This means you’ll still have to go the MyClass.classVariable = 42; route for class variables.
If you just want to initialize a class with some defaults, there is a rich new syntax set for function argument and destructuring defaults you can use. To give a simple example:
class Foo {
constructor(foo = 123) {
this.foo = foo;
}
}
new Foo().foo == 123
new Foo(42).foo == 42