This should probably be a suggestion on Github, but shouldn’t #oembed​s from e.g. #Flickr also support portrait mode similar to uploaded photos?

For example https://mastodon.social/@aslakr/110904032874948549 only shows square preview. Even square photos https://mastodon.social/@aslakr/110818611849002112 are shown as a rather small preview while landscape isn’t https://mastodon.social/@aslakr/110930242375794631