Agree; now that I think about it, long term a really cool solution would be for WordPress to adopt more robust embedding support (a la Onebox) natively for any external site with the requisite opengraph data. But I know that’s a whole separate discussion on the WP side 
I do think a more limited solution with WP Discourse, enabling topic blocks to embed a post, could still be pretty useful for site with tightly coupled WorPress + Discourse sites that reference each other often. For the use case I imagine it’d almost always be me wanting to embed a discussion from my own Discourse instance on a WP page.
Block-type replacement for the kinds of shortcodes you mentioned sounds cool @simon, will keep an eye for something like that! (Or depending if you’ve already started looking into this, maybe I’ll try hacking on this at some point as an excuse to check out the new WP editor + learn JS in more detail)