Such a short signed message could be used out of context in the future. Bob somewhere else is talking about a different idea, and I claim to be Alice and send A

Such a short signed message could be used out of context in the future. Bob somewhere else is talking about a different idea, and I claim to be Alice and send Alice's signed "this is a great idea" message. The signature checks out, and it seems to be in response to what Bob is talking about, so Bob would assume Alice think his idea is great. You want to make sure that whatever you signed can't be used against you out of context. It would be safer to sign a message like "date> {today's date}, re> {parent message hash}, this a great idea"