thank you for your feedback. npm support was a big win for us since we tried to get the dependencies (especially on languages and package managers) simple.
we definitely will keep sass/scss in mind since it is very popular in the rails community. once the core components work well together we will look into customization (also for the templating component).
that said, I believe there is value in a recommended set of components to have consistent documentation, code examples and a straightforward getting-started experience. but for people who know what to do it should be easy to switch components (comparable to replacing activerecord in rails). easy things should be easy, hard things should be possible :)
That said, I'd definitely like to see some integrated support beyond Stylus for things like SASS, LESS, and/or Compass.
Keep up the good work!