If someone enters a message and you forget to quote them, there should be an option to select a message you wish to quote in the chat history from other users.
This will save users the pain of having to copy their message to the clipboard, delete their message, paste in their original message again and then select the user they wish to quote. I see that happen literally ALL the time and it would be an absolute leap forward over all other competing chat programs on the market.
Here is a crude example I have drawn up for a better understanding. Of course, once the chat has been selected (IE: where the cursor is on the blue selection in the second image) that message will then trigger a pop-up dialog to confirm the quote pair (a crude mock-up image of this is on my second post).
Better wording than "Quote Message" in the drop-down pane might also be "Pair Quote" or "Quote Into". I'll leave that up to the Discord devs to decide what makes the most sense.
10/FEB/23: "Quote Into" seems to make the most sense, and once clicked, you would only need to then select 1 message unlike in my image how I have 2 being selected (IE: green and blue). In my example, it would be green being selected to trigger the pop-up dialog in my second post as you cannot quote yourself.