This isn't quite an answer to your question, but my approach is to write the message first. Usually by then the subject is obvious.
But for where it isn't, I generally go with something like "A question" or "A thought" or "Some possibilities" or something equally neutral like that.
This thread is for discussing anything that doesn't seem to deserve its own post.
If the resulting discussion becomes impractical to continue here, it means the topic is a promising candidate for its own thread.