Hey! I'm part of the small team building Superpowers over the past 6 months. I've been working on collaborative game-making tools for several years and Superpowers is the result of a lot of that work. I'm very proud of the small core + plugins architecture we came up with and excited to open source (MIT) it all soon!
We evaluated various possibilities and even started building our own scripting language (because TS compiler APIs weren't ready back then) before going with TypeScript. Many reasons:
We want Superpowers to be as "Web native" as possible. TypeScript is just JavaScript + types, they're working to align with ES6 and maybe even merge with ES7+, they have a lot of mindshare, their compiler services are awesome (having intellisense-like in the editor is a big deal) and the whole compiler can be run in the browser, which means it'll work everywhere just like the Web we're building on.
I'll be happy to answer any questions, obviously.