For an opinionated framework like Ruby on Rails, I expect it to come pre-bundled with a CoffeeScript based test framework and test runner. Here’s why:
Teams don’t need to waste time deciding which one to use among the mess of Jasmine, Mocha, Expect, Sinon, Chai, Bower, Grunt, Teaspoon, Npm… you name it.
It’d work better than what’s out there since the Rails team will focus on clean integration with Rails and will have a much larger user base to collect contribution from.
I’d prefer Jasmine to be the test framework and a runner similar to teaspoon that can run the specs both on the command line and on the browser, with asset pipelines enabled.
Rails made a huge promise to test automation by shipping with a nicely structured way to test the ruby code and doing the same for JS would make life better for so many people like me, I believe.