Several buildings from events require an event item to upgrade. After events it's impossible to upgrade for a year. Many events I have had some extras but not enough at one time for those upgrades. How about turning left over event currency items into Holiday coins? The Holiday coins could be collected and traded all year long and used to upgrade those event buildings which require special event currency while still getting rid of event currency after an event is over. The holiday coins would not be able to be used to purchase event items, only the special building upgrades.