Add an option to the "ended:" messages for losing the duel.
Configurable reward in config (true/false)
More configurability to the messages.yml. i.e. ability to change all messages
If not the above, just the option to adjust the color of the currently uneditable messages (I've seen other plugin do this as part of editing the prefix, e.g. "&7[&3Duel&7] &a")
The ability to cosmetically change the /duel, /duel list /dueladmin, etc menus