When creating a new page from an SPFx full-page enabled web part, the developer cannot provide an image to the page template.
Also the preview renders the web part as a new web part with no properties in the preview page - it would be great to have a context-flag telling the developer to render with sample data so that the preview will be a fully-configured web part and not an empty web part.

Most web parts render a stub message when not configured, like "please select a list" or something like that, so the preview isn't helpful at all.