I love the threaded messages in the cards view but there is no indication that a new message has arrived in the thread unless you expand. Would be great if the card turned bold again to indicate there are new messages in the thread.
YES! I have the same problem or idea respectively. It IS a problem that a new email within a thread is not obvious. (Maybe the thread moves upwards with a new email in the thread, but what if other emails arrive, too? You will easily miss the new "threaded" email.)
Here's my suggestion: make the "arrow" (>) that indicates a thread BOLD as long as there are new or unread messages inside. If all the messages have been read, it will turn to, well, normal, thin, you get the idea.
I would love to post an image that I prepared, but I don't know how. Apparently you can only LINK to images. Well then, I hope the explanation will do.
This, along with more sensible handling of repeated messages with the same subject heading, is THE issue that keeps me from completely converting from using the Gmail web interface to using Thunderbird.
I agree with @nowakomm's solution that the thread should remain bold as long as there are unread messages within it. I would actually consider this a bug, rather than a feature request.