Similar to our news app template, I've put together a Grunt scaffolding for creating bundled custom elements, including HTML templating and CSS, all in a single standalone file. It's our component template — or, as I like to call it, the Poor Journalist's Polymer.
My second testbed project is a Leaflet map element that uses custom HTML to set the map configuration without ever writing a line of JSON (unless you really want to). It's intended to make mapping simple and fast for web producers, while still offering plenty of power for people like me who just want the boilerplate out of the way. Leaflet's a great candidate for this kind of declarative approach, and I think this is a really promising demo for the power of custom elements.
For standalone components like these, the template seems to be working well. I haven't yet solved the problem of easily embedding them in highly opinionated news apps, due to the way that dependencies are handled. It's useful for custom elements to be able to bundle their CSS and other assets into their package, similar to the way that HTML imports and shadow root offer embedded styles, but that means they may not integrate well into projects that already have their own build system. As far as I can tell, the best solution for now will probably be to load the packages from Bower and require() the standalone files from its build directory, which should work with whatever module system you like.