I think it would be nice to have the stage 1 private fields proposal implemented in TypeScript. It'll mostly supercede the current pseudo-private implementation, and a fully-correct implementation is only transpliable to ES6 using per-class WeakMaps. I'll note that the spec itself uses WeakMaps internally to model the private state, which may help in implementing the proposal.

Currently, the proposal only includes private properties, but methods are likely to follow. Also, the most important part of the proposal is that private properties are only available within the class itself.