Typescript. I imagine most people writing Rails applications are also writing typescript for front-end code, so being able to use the same muscle memory for Ruby typing seems high desirable. That is the thing that stood out to me when I saw this site: it looks like they are taking the very positive lessons from Typescript and applying them to Ruby.

I agree with other posters here. I don't need everything typed - Ruby's duck typing is an awesome feature - but I do wish that some of the more important interfaces in our code were more strongly self-documenting and enforced.