Look at those functionality tables. Even if node4 is LTS etc... if the implementation of ES6 is not full, you may have not access on all ES6 functions yet or some functions will be change along the ES6 implementation, so there is a risk of unstability when coding on ES6 at the moment.
I agree with you about use ES6 but not for a framework at the moment because of engines that don't support it at 100%.
You can create it, develop it, making it grow but, don't ask people to put it on production for the moment.
I love SAILS and I love ES6, i think your project will have a future, i will watch it closely and as soon as node4 will be at 100% of implementations , I will use your framework, making some proof of concept and enjoy it.