Apologies in advance if this has already been suggested.
I noticed that there were some themed clothing and weapons for the Trick or Treat event, some of which I presume is only available via purchase with Real World Money.
My suggestion (not necessarily Trick or Treat themed, could be any event) is to have expendable themed items that actually have noticable advantages during the event.
As an example, for a Trick or Treat themed expendable item, I was thinking of some kind of Temporary weapon, like a Pumpkin bomb or grenade. Maybe this particular weapon could increase the chances of treats dropping more than even the current themed weapons.
Since it is an item that is a one time use that has event impact, I would assume it would be highly valued (more than current themed clothing/weapons), and could be Real World Money monetizable by the devs.
Again, this doesn't necessarily need to be just for Trick or Treat, but this idea could be used for any event.