A litmus test I'm excited about is writing a less clunky linear algebra / numerical optimisation package.

Gonum is good, but it can feel like a lot of the powerful abstractions in numpy and Eigen are difficult to replicate.

Whilst numpy and Eigen do "turn the magic up to 11" just a bit too much for my liking, I do like the idea of things like this:

    interface Scalar() {
        type float float32 int int32 int64
    }

    type DenseMatrix(type T Scalar) [][]T

    func Identity(type T Scalar)(int size) DenseMatrix(T)

etc.