Test for build failure error messages, such as: css syntax error when not using preprocessor
Better compiled output without multiple "use strict" and other nonsese.
Make one demo page that lets you write codez that are shared between es5 and class-based components so you don't have to duplicate stuff.
Support extending things other than HTMLElement. https://blog.revillweb.com/extending-native-dom-elements-with-web-components-233350c8e86a
Have the yeoman generator offer options for TS or ReasonML or whatever JS setup.