A very nice addition to TypeScript's type system would be sum types in the spirit of ML-like languages. This is one of basic and simple programming constructs from functional programming which you really miss once you get used to it, but which seem to have a hard time being included in new modern languages (contrary to other features from functional programming such as first-class functions, structural types, generics).

I guess the most natural way to integrate sum types in the current language syntax would be to extend enum variants with extra parameters (similarly to what Rust does: http://doc.rust-lang.org/tutorial.html#enums ) and upgrade the switch statement to a more powerful structural pattern matching (although in a first step, simply discriminating on the toplevel variant and capturing its parameters would be already quite good).

This is quite different from other the proposal about "union types" (#14), which would mostly be useful to capture types in existing Javascript APIs. Sum types are rather used to describe algebraic data structures. They would be particularly useful for any kind of symbolic processing (including for implementing the TypeScript compiler).