I have two ideas:
Add Emoji / GIF reactions to sent messages.
(like dc or fb chats, i think this is a popular feature)
Possibility to create a vote.
(also in chat, although a server level voting can also be a good idea )
I would suggest a button that can be used to reply to a given message and show which one you have replied to, as in messenger
(Because busier / bigger chats are chaotic and it can be hard to figure out who replied to whom and which message)