It would be great if the "Replay" button was still shown even if the current quest is 100% completed. Many times we'll go back and clear a quest we've already done to grab additional items or get points for things like completion or hero use. Currently at the end of the quest we have to exit out, navigate back to that quest's location, and re-enter the quest.
It could replace the "Upgrade Champions" button as it's rarely used and easily accessible from the main menu. Ideally whenever a quest ends we would have "Exit", "Replay", and "Play Next" always available.