Something cool I thought of is adding events to your game, once complete. Think of it like this.
On Halloween, you add a boss battle in your game, killing it sets a number 0 to 1, filtering you unlocking a Halloween exclusive item.
Even if you delete the boss, after the game saves, the 0 will always be a 1 for players that unlocked it. Nobody else can unlock that item after Halloween.
The item will remain in game, but the boss does not have to, because the boss is the number trigger and the number is saved at 1.
It doesn't have to be a boss, anything can trigger the event.
Use this idea if you finish your game or want to give out beta tester exclusive items.