The most useful would be the ability to customize the OG image tag.
Having a global default would be convenient and then having a per-post specified image would allow us to be sure the right image appears when posting to Facebook or other social media. This is a very common and annoying problem with Facebook links to forums and articles: FB often displays the wrong image when the OG image tag is not specified.
Usually, the other tags that are already there (url, type) would remain the same as they are now, but customization would be very powerful for optimizing a post's appeal to particular groups/pages in social media.
For example, if posting the same post to 2 FB groups, one about science and one about history:
For the science group, the OG title and description tags would default to the values they currently do now, but they could be customized to emphasize the scientific information presented in the article.
For the history group, the OG title and description tags could be customized to emphasize the historical significance of the article.
This would be quite powerful to widen the appeal for a particular post/article when it contains info that appeals to different groups of people, which they often do.
