Will be using TypeScript. For internal implementation only. Does NOT affect ES users, but should improve TS integration for TS users.
An example of this is how in Redux, anything that uses the state in TS needs to add an annotation to type the store, because its completely decoupled (and thus theres no way for the state type to "follow"). Any way to make it follow requires some API changes (eg: creating mapStateToProp functions by calling a method on the store)
Most Angular devs I know are happy to use TypeScript. (In fact going out on a limb, most JS devs I know).