The idea has been discussed. The main problem is that Mastodon is a zero-trust environment, and we can't trust the other server to send us the correct preview.
Don't trust the server, trust the poster/client... that's really where the preview should be coming from anyway. And if you _don't_ trust the poster, you shouldn't be trusting anything they link you to anyway. It's trivial to lie to these preview generators with a couple of meta tags if the destination isn't trustworthy.