The material design spec on chips has a section that describes "input chips": https://material.io/design/components/chips.html#input-chips 

This is a really common usage and we should consider implementing this spec for a chops-chip-input element that allows people to enter free form chip data. This could be a more general replacement for our current chops-user-input element.

I'd like to do this within the quarter as part of trying to create a new Polymer Monorail form.