I like the idea, though I don't know if it's suitable for this extension or a better fit for a new one. I guess a 'synopsis' could be as general as the author wanted to make it, and potentially control multiple areas where summary posts 'could' be used such as discussion preview, user profile (posts & discussions), and even best answer OR it each could be handled in each individual extension but I guess the behavior and display options between them might not necessarily be consistent. I'm with you though as I think the other areas mentioned would be a nice option to have excerpts for... for example have Best Answers be limited to a preview rather than full post as I find it to be a bit too in your face when the post could be any length (1000 chars in the second post is a lot! even more so when the first post is small and it doesn't look quite right), especially when you can simply click it to take you to the full post.
Thanks for this extension, I saw the talk in the other thread about the last post option and also thought that would be a nice addition so this extension is going on my forum 🙂 The flarum team and yourself have delivered countless updates and extensions for the holiday season - thank you for the excellent work!!