I suggested:

> Or, complete a face name: you see the faces whose names you can
> choose (like `list-faces-display', but with completion). WYSIWYG.

Attached is a screenshot, to show what I meant. The only change needed was
to make `read-face-name' apply the face to the face-name string.

It might not look pretty, but the result conveys more info than do just the
face names. The point is that *Completions* can usefully take advantage of
text properties - this is one example.