While using ytt is great, it is not as well known as helm and in some cases sticking to helm reduces complexity and makes it easier to introduce myks. Therefore, we could introduce a new folder type in prototypes that contains local charts, e.g.:

prototypes:
  my-app:
    helm
    vendir
    ytt
    **chart**

The chart folder would contain a local helm chart in the standard format:

chart:
  templates
  Chart.yaml
  Chart.lock
  values.yaml

When rendering, the local charts would be templated and added to the remote chart (if there is one).