I think the most useful messaging feature would be one that tells you how your message might actually be received.
You know what you mean when you type something, but the other person only sees the words on their screen. A joke can sound rude, a short reply can sound cold, and a perfectly normal question can accidentally come across as passive-aggressive. Before sending, it could flag possible misunderstandings without rewriting your entire personality into something polished and unnatural. Would you trust a second opinion before sending important messages?