This is a small DX improvement so we can cleanly group related components, I'm thinking to allow the discovery of index.html inside subdirectories. To demonstrate:

Say I have a sidebar component. And in the sidebar I have a block, title, link. The current way I have 2 choices:

templates/
  sidebar/
    sidebar.html
    block.html
    link.html
    title.html

Con: This requires us to use <c-sidebar.sidebar /> for the main component.

Or

templates/
  sidebar.html
  sidebar/
    block.html
    link.html
    title.html

Con: This scatters component-related files around.

Suggestion:

templates/
  sidebar/
    index.html <--
    block.html
    link.html
    title.html

This allows us to access the main component like <c-sidebar /> whilst still being able to access its sub components like <c-sidebar.title />