Being able to set reminders for messages received, whether it be a group chat or private. As simple as hovering over the message and maybe clicking a drop down box (where other options such as edit, save, or delete message would be) then clicking "set reminder" and then just set a desired time.
Combined with the list one can create 'saving' messages by marking it with a red ribbon this could be a perfect ToDo functionality.
After a message was saved, there should be options to set reminders to self/channel/team that can trigger once/n times/regularly.
Also the ability to mark/prioritize items in that list would be great.